The Doctor knew that Nyssa would admire the beautiful landscape, even though it wasn’t a conscious choice to come here. The TARDIS was brought here by a gravitational anomaly, but the Doctor’s confident they’ll have no trouble leaving as long as they give the ship enough time to regain her strength. In the meantime they’ll have to make the most of their time here. The Doctor had always thought of Vortis as somewhere he could relax, but crises have always got in the way during his previous visits. That seems to be the story of his life, even though he insists he doesn’t often go looking for trouble. He tells Nyssa about the inhabitants of Vortis, in particular the Menoptera, a sentient species of moths/butterflies about the same height as her. But if she thinks that sounds big, just wait until she sees the Zarbi.
He realises there are more trees here than when he last visited a few regenerations back and he estimates it would have taken hundreds of years for the forest to grow back this much over the entire planet. Nyssa wonders if they’re arrived before the time of his earlier visits, but the Doctor points out there are several moons that weren’t always there. In fact, the moons arrived in very specific circumstances which means it’s definitely after his time. Nyssa suggests the planet may be in a great orbit lasting thousands of years and perhaps the trees are only blooming now because they’re moving out of Winter and into Spring. The Doctor says there are some parts of the Universe that have bred the most wonderful mysteries and he spends most of his life trying to unravel them, but sometimes he thinks it’s nice not to know everything.
Their conversation is suddenly interrupted by the sound of insect chirruping which soon becomes a cacophony of noise. Before either of them can react, the Doctor is knocked to the ground and winded by a huge Zarbi which races past them. Nyssa theorises that it must have been a scout of some kind, because close behind it, an entire army of giant termites is racing towards them. The Doctor says they’re harmless unless someone gets in their way, but there’s no way they can avoid such a swarm, so they both run for their lives. They soon realise it’s no good and that the advancing Zarbi are much faster than them. They risk being trampled underfoot so Nyssa suggests they try climbing a tree and the Doctor congratulates her and offers to help her up. Unfortunately the nearby trees are all covered in a kind of goo and Nyssa finds it very difficult to hold on. The tree won’t hold the weight of both of them, so the Doctor starts climbing up the next one along. As the Zarbi catch up and swarm past, some of them bump into the trees and the goo causes Nyssa to slip. The Doctor pleads with her to hold on, but it’s no use and eventually she falls to the ground. The Doctor is about to leap down to protect her when a huge winged creature swoops down from the air, grabs hold of him and yanks him up into the sky.
Nyssa looks up, half conscious, and calls out to the Doctor, but then a strange man appears from nowhere and orders the Zarbi to stay away from her or else he’ll burn them. Nyssa realises the man is human and asks who he is, but he calls to her to stay where she is and says he’ll come back for her. As he leads the Zarbi away, Nyssa tries to move, but her head is still thumping. Then she hears a female voice, speaking to her and telling her she’s something new on this planet.
The Doctor calls out to the winged creature that carried him away and orders it to put him down. The creature is an elderly Menoptera who speaks to the Doctor in a soft voice and reassures him that he won’t drop him. He says it’s just a short flight to his Tower, but the Doctor insists on being returned to the forest where he left his friend at the mercy of the stampeding Zarbi. The Menoptera introduces himself as Acheron and says his daughter is in the forest too, but unfortunately he’s only able to rescue one person at a time.
The young female Menoptera helps her new friend stagger through the forest, but Nyssa is barely able to stand and wants to rest for a while. She asks if it’s far to her home and the female says it would only take a few minutes to fly to the Tower, but it’ll take an hour to walk. As she sits down, it occurs to Nyssa that she may have cracked a rib. The female says her father will be able to help as he already knows something of the anatomy of Sailors, even though he’s never studied a living one before. Nyssa queries the term and when the female speaks of the Sailors of the Great Ocean Between the Moons and Beyond, she realises she’s referring to space travellers. The female tells Nyssa she finds her strange because she has a new scent and is very ugly, but when Nyssa takes offence, she realises Nyssa doesn’t know what it means to be beautiful amongst the Menoptera and she admits that she is ugly too. Nyssa is confused because the Doctor described the Menoptera as butterflies, but her new friend evidently has no wings. She decides she really ought to find the Doctor, but the female suggests she waits until she’s feeling better. She introduces herself as Hedyla, but she struggles to pronounce Nyssa’s name and the best she can come up with is ‘Anylsa’. She assures Nyssa that the Zarbi are harmless unless you blunder into them and they come from a wild hive in the Desolation beyond the edge of the forest. No one knows what they want so her father came here in order to study them. When she says no Sailors have ever survived the wrecks, Nyssa realises they get a lot of spaceships crashing in the area. Hedyla knows of some caves nearby, so she suggests they shelter there as the Zarbi are likely to return soon.
The elderly Menoptera Acheron lowers the Doctor to the ground and welcomes him to the family Tower house. From this high vantage point they can see right across the forest of the Desolation and can see the black line below the trees that is actually a swarm of wild Zarbi. The Doctor asks if the stampede is normal and Acheron says the Zarbi have always foraged in the area, but never before in such huge numbers. It’s particularly curious because they don’t seem to feed and they take nothing from the forest. The Doctor is worried about Nyssa, but Acheron thinks he can help and leads the Doctor to his laboratory. He pulls out a device like a harp, which he calls a Summoner. He devised it himself in the hope that it would be effective against the Zarbi raids. He invites the Doctor to tear one of the membranes strung between the frame as hard as he can. The noise the device makes sounds just like a Zarbi and Acheron explains that the membranes hold recordings of wild Zarbi mating songs and it acts like a siren, calling them back to the nest. They should be able to hear it right across the forest, but unfortunately he’s never had the chance to test it. As there’s no time like the present, he asks the Doctor to pluck away at the Summoner…
Inside the cave, Nyssa and Hedyla notice the Zarbi are behaving oddly. When they start leaving the area, Hedyla realises her father must be responsible and she tells Nyssa about the Summoner. Once their route is clear, they decide to continue their journey to the Tower. They have no idea how long it will take the Zarbi to discover they’ve been deceived, so they set off immediately.
The strange man who called to Nyssa earlier in the forest returns to the area he last saw her. He bemoans the fact that Xanthe got away even though he told her to wait. He races off to look for her…
At the Tower house, the Doctor examines Nyssa and concludes that she has a few bruises and probably pulled the odd muscle, but there seems to be nothing broken as far as he can tell. In a few days she should be back to normal and as the TARDIS isn’t going anywhere soon either, they decide to wait for her to get better. Fortunately Acheron is delighted to have their company and Nyssa has found a new friend in Hedyla. The Doctor thinks the Tower is a very unusual choice of home for a Menoptera as they normally build their nests in treetops, but this one can be accessed from the ground. Nyssa assumes it was built specifically for Hedyla as she’s wingless, which the Doctor says is also quite rare among Menoptera. He also thinks it’s curious that they’ve decided to live on the edge of wild Zarbi territory. Nyssa asks whether there’s any humanoid life on Vortis because she thought she saw someone when she was lost in the forest, but before the Doctor can respond, Hedyla joins them and invites them to dinner.
Later, the Doctor and Nyssa thank their hosts for such a ‘delicious’ dinner, which surprises Acheron as he noticed they only digested their food once. He asks his daughter to bring some drinks for their guests - but not the acids - and then tells the Doctor and Nyssa how much he’s enjoying their company. He thinks there’s so much they can learn from each other and he admits that so much knowledge about his own race has been lost over the years through ignorance. The Doctor notes that Acheron left his own people to continue his studies and he acknowledges that he’s chosen a hard path. Acheron says that everything they have here has been provided by nature or by the Zarbi - even the Tower’s walls are made from congealed Zarbi paper. Nyssa wonders why he can’t continue his studies of the Zarbi while living among his own people, but Acheron says they regard the Zarbi simply as a workforce, a burden or a menace. Research isn’t encouraged, but not just into the Zarbi. Acheron believes the Menoptera have been rigid and closed-minded for a long while. He points to the moon Pictos and tells them that their ancestors were once exiled there when the Dark Power invaded, but the occupation has left scars that persist among his people to this day. Pictos is a permanent reminder of that time and it’s turned the Menoptera into militant reactionaries who fear anything new. Nyssa points out that the Zarbi are starting to become militant too, which is puzzling Acheron. He says he’d like to go into the Desolation to investigate, but he’s reluctant to leave his daughter alone. Hedyla points out that with ’Anylsa’ resting here for a while she won’t be alone, but Acheron is still unsure whether he could make the trip to the Desolation on his own. The Doctor volunteers to accompany him, but as he can’t fly he’s not sure he could keep up with him. Hedyla says they have another means of transport he could try…
Sometime later, Nyssa watches as the Doctor nervously mounts one of the Zarbi and sits astride it like a horse. Hedyla assures him the creature has been fully trained and she’s ridden on it herself many times and the Doctor accepts that things might be different if he too had months of practice. Unfortunately the distance to the Desolation is too far to walk and he’d never be able to keep up with Acheron on foot. Nyssa is sure the Doctor has tried stranger forms of transport, but he can’t think of any offhand. Hedyla has named the Zarbi Arbara and says she likes it when people scratch her antennae. Acheron lifts off into the air and tells the two women they’ll be back shortly, bearing knowledge from the forbidden land. The Doctor says goodbye to Nyssa and then begins to ride Arbara into the forest.
Hedyla invites Nyssa to view their progress from the lookout post at the top of the Tower. It’s a long way up but Hedyla says her father likes being close to the air. The view is very impressive, although it’s precarious too so Hedyla attaches her to a safety line. They look out across the trees to the Desolation and Nyssa is surprised as she was expecting it to be barren, like a desert. Hedyla says there are some bare patches where the ships fall from the ’Ocean’, but natural desert has been rare on this planet for ten generations. In the distance behind the Tower is the edge of the Menoptera city and Nyssa can just make out some tall spires which Hedyla says are the tops of the Temples of Light. Their people worship light itself and believe it to be sacred, and some even believe there are gods within the light. Nyssa has noticed how different Hedyla is from her father - he is very expressive, making big gestures with his voice and hands, whereas she is softer and gentler.
Just then, Nyssa notices more Zarbi in the distance - thousands of them in a swarm and they’re not coming from the Desolation. Hedyla explains that they’re not wild Zarbi. They’re undergoing a Migration and they’ll cut through the forest, consuming everything in the path, before moving on. Hedyla is confident the Doctor and her father are in no danger as the Zarbi won’t venture into the Desolation. They’re beautiful in a way but they destroy everything they touch, yet the Migration takes so long the forest always has a change to re-grow before the next one. Nyssa has guessed that Hedyla wasn’t born without wings and the Menoptera girl confirms that she was beautiful once. Captains and Princes flocked to see her display and she could have been a Queen. Unfortunately, her father was notorious for having unpopular opinions that he voiced loudly, so he made enemies. Some of his opponents had followers who were prepared to hurt her father by any means and one day Hedyla was foolish enough to be lured by lights and bright colours… Most of the perpetrators were caught and some of them even expressed remorse for what they’d done, but their aim was to silence her father and drive them away, and they succeeded. Hedyla asks Nyssa if she’s strong enough to climb back down as there‘s something secret in the forest she wants to show her.
The Doctor and Arbara are becoming quite fond of each other and Acheron suggests he take her with him when he leaves, but the Doctor isn’t sure he could cope with having a Zarbi in the TARDIS. In any case, he still has to find out what it was that drew the TARDIS to Vortis in the first place. He thinks the answer is nearby as some of the crashed spaceships must have been here for hundreds of years. He thinks it’s unlikely to be a natural phenomenon or else the ships would be spread out across the whole of the planet rather than concentrated in one spot. He’s also realised the flora in this area is different to that anywhere else, and although it could be the result of mutation caused by leakage from a faulty fuel cell, he thinks this is unlikely. Although Vortis is a fertile planet, it seems richer here than anywhere else. It’s almost like an experiment, so this could be the testing bed for new forms of life.
Acheron explains that the name of this place, the Desolation, comes from the Dark Times and refers to Pwodoruk, the creator of the soil and the darkness from one of the Menoptera’s older religions. The Light made the Universe and the sky, but Pwodoruk made time and the ground so that things might wither, but also grow. Acheron tells the story of Hruskin, the first Menoptera and grandmother of all others, who went to Pwodoruk and told her about the world she imagined for her children, where they would build beautiful temples for the Light and dance and weave songs. This world would be paradise and no Menoptera would ever want or suffer. In response, Pwodoruk asked Hruskin who would build the temples and raise the harvests and do all the work while all the Menoptera were free and at leisure. Hruskin suggested they have slaves, so Pwodoruk created the Zarbi.
Suddenly something disturbs Arbara and she becomes agitated and starts running for home. The Doctor and Acheron are thrown off their feet when the ground itself splits apart and swarms of Zarbi burrow their way to the surface. The Doctor realises they must have deliberately buried themselves and lain in wait to ambush them. Acheron tells the Doctor to stretch out his hand so he can fly them both to safety, but before he can do so, the Zarbi grab hold of Acheron’s leg and drag him back to the ground.
Hedyla explains to Nyssa that the natural habitat for the Menoptera is the sky and no Menoptera would ever dare venture below ground, even her unconventional father. Nyssa realises she’s being taken back to the caves and although Hedyla admits that this frightens her, she knows her father would fear it more. As they enter the caves, Hedyla reveals that according to their legends from when the Menoptera were exiled from Vortis, most of her people went to the moon, but some of them elected to flee underground instead. There, the darkness and the earth changed them into timid things and they were never able to change back. They reach their destination and Hedyla lights the tunnel to reveal that she’s constructed herself a pair of giant wings. She says she could never display them as Zarbi paper is too grey and ugly, but she often imagines that if she wore them on the roof of the Tower and jumped off, she could fly. Nyssa is amazed at how light they are, but they’re also sturdy and aerodynamic and she thinks they might actually work as a glider. Nyssa says this is the dream of all flightless creatures and she encourages her friend to try her new wings. Hedyla is worried that her father would find out, but more importantly she wouldn’t really be flying, just pretending. She begins to cry and asks Nyssa if she can wait outside for her.
Outside the cave system, Nyssa is sad for her friend, but her thoughts are interrupted by the return of the strange man she saw earlier. For the first time, she notices something strange about his face. He claims to have been searching everywhere for her and says he’s been confused and lost without her. He says his name is Yanesh and he evidently believes her to be someone called Xanthe. He says he can’t be healed without her and shows her his hands and face which are covered with alien plant life. He believes she can cure him and he reveals that the vegetation is completely covering his entire body and is actually growing into him. Just then, Hedyla joins them outside and she recognises Yanesh. He panics when he sees her and flees back into the forest. Nyssa asks Hedyla what she knows about the strange man, but she simply tells her he’s another new thing.
Acheron is starting to panic as the Zarbi drag him and the Doctor deeper below ground, believing that if they go any deeper, he’ll become a pitiful thing. The Doctor has a plan and tells Acheron that because the Zarbi have no concept of pockets, they didn’t bother to search him. The Zarbi bring their prisoners to a halt as something comes towards them and Acheron is shocked to discover they’re facing a human female. The woman introduces herself as the Speaker for Mother Life and the Doctor realises she must be a survivor from one of the crashes. She explains that she’s speaking on behalf of Mother Life, who is all around them and Acheron deduces that she’s referring to the whole of the Zarbi hive itself. The Doctor wonders just how long the Speaker has been here as she‘s become seamlessly integrated into the hive-mind. He asks why they’ve been brought here and the woman says the lode-seed has been taken from Mother Life and it must be returned. She says the lode-seed makes the soil rich and draws down nourishment from the overland to create new life. Acheron believes the overland she refers to is actually the Great Ocean - or space - and the Doctor concludes that it’s a powerful gravitational attractor. This would also explain the crashes. The Speaker explains that the land is hungry and needs fresh seed and it’s Mother Life who sifts out the healthy soil. The Doctor compares it to a galactic compost heap that attracts space detritus, but it also captures any ships that come into orbit, including the TARDIS. The Speaker orders the Doctor to bring the lode-seed back and he agrees, but he adds that she doesn’t belong here in the tunnels and she should accompany them. The Speaker refuses and orders the Zarbi to take the two prisoners back to the surface. The Doctor warns Acheron to close his eyes, then he ignites a ball of light that momentarily distracts the Zarbi. They grab the struggling woman and despite her protests, they start to drag her back down the tunnels the way they came.
Nyssa wonders how long Yanesh has been on Vortis and Hedyla reveals that it hasn’t been long. She regards him as a “vegetable thing” but Nyssa is sure that’s not his natural state. She’s certain he’s been infected by something that’s growing inside him, but only the Doctor will be able to confirm this.
Outside the hive, the Doctor and Acheron stop for a rest and lay the Speaker down on the ground. She hasn’t said anything for a while now, but the Doctor is sure there must still be a part of her that understands him. Acheron advises they give her time to recover as being removed from the hive must have been traumatic. He suspects the hive will want her back and he says he’d be very surprised if there weren’t scouts looking for them already. Just then, they see a Zarbi approaching them, but fortunately it turns out to be Arbara, who must have followed their scent. This solves their problem and the Doctor suggests Arbara carry the Speaker on her back, which will free up Acheron to fly up and find them a safe route back to the Tower. The Doctor admits that he’s concerned about how easily the Speaker was assimilated into the hive. Her clothes are dirty, but not ragged, so unless the Zarbi have a sideline in launderettes, she’s probably only been here for a few weeks. Acheron wonders if she may have come from a human hive-mind, but the Doctor says they wouldn’t be compatible and if anything, it would have made the assimilation even more difficult. Acheron then suggests she might travel in the Great Ocean alone and being an individualist might have made her more vulnerable. He offers the Doctor the use of the facilities in his laboratory and says he’s confident that even if the Zarbi follow them back, they won’t attack for fear of damaging the Speaker.
As the Doctor and Acheron return to the Tower, they’re joined by Nyssa. The Doctor helps the Speaker down from Arbara and asks Nyssa to help him carry her to the laboratory. On the way, Nyssa tells Acheron that his daughter is on the roof and wants to speak to him about something very important. As they move inside, Yanesh watches them from his hiding place not far away. He slowly approaches the Tower, promising to get Xanthe out of there…
Nyssa helps lay the Speaker down on a table and observes that she seems catatonic. The Doctor thinks this must be separation trauma so he tries to reassure the woman that no one is going to hurt her. The Speaker begins talking again and tells them the Mother Life needs to retrieve the lode-seed. The Doctor promises he will do everything in his power to find it, but first he says the hive must let her go. He tells her to relax her mind and then asks who she was before she became the Speaker. The woman says there was no need for a Speaker until the lode-seed was taken, but she seems unable to answer any questions about her life before joining the hive. The Doctor is surprised when Nyssa announces that the woman’s name is Xanthe and says she’s spoken to another man in the forest whose name is Yanesh. He spoke of Xanthe and since there can’t be that many human women on Vortis, it seems likely this is her. She also says that Hedyla has seen the man a few times since the Zarbi raids began but that he‘s been infected with some sort of vegetation and there was a large seed, about the size of a fist, growing into his chest that made a curious humming noise. The Doctor realises this must be the lode-seed, but he doesn’t want the Speaker to know about it just yet so he asks Nyssa to stay here and look after her while he pops up to the roof to speak to Hedyla.
Some time later, Nyssa goes to the top of the Tower and is surprised to find Acheron there alone. He seems angry and says the Doctor and his daughter have gone and they didn’t leave a message to say when they’d be back. He peers over to look at the migrating Zarbi down below and observes that they’re changing course and turning inwards back to the forest. Nyssa realises that they’re heading for the Menoptera city and if they don’t warn his people, they’ll all be killed. Acheron is dismissive of her concerns and says the Menoptera are proud of their own defences and there’s nothing he can do to stop the Zarbi even if he wanted to. Nyssa suggests distracting them with his recordings of the Zarbi mating calls, but Acheron assures her they’re a different subspecies and it wouldn’t work. Nyssa says he could fly to the city and warn the Menoptera personally, but he sees no reason why he should do that. Acheron reminds Nyssa of what his people have done to both him and Hedyla, but Nyssa says that’s just petty revenge and isn’t a fit attitude for a man of learning. She pleads with him and eventually he agrees to try the recordings, even though he’s sure they’ll do no good.
Hedyla has taken the Doctor to the area of the forest where she’s often seen Yanesh, but she says he always runs away from her so she doesn’t know where his actual nest is. The Doctor thinks the man must be starving, lonely and afraid, and if Nyssa is right about his infection, he may even be dying. He thinks the man must be the key to everything. Yanesh and Xanthe must have taken the lode-seed, perhaps to study or even destroy it if they thought it was the cause of their crash. It wouldn’t have helped them escape, but it would have spared others the same fate. Hedyla wonders if the Zarbi would kill him to get it back and the Doctor concedes that Mother Life has already absorbed the poor woman Xanthe and there’s no guarantee he can help her. It occurs to him that the names Xanthe and Yanesh represent the letters X and Y and he suddenly realises he’s made a huge mistake. He insists they get back to the Tower quickly…
Yanesh is surprised to find the walls of the Tower are made of paper, so he has no trouble burning his way inside. He finds the Speaker and tells her he was afraid he’d never see her again and as she regains consciousness, she addresses herself as Xanthe. Yanesh apologises for leaving her, but says he was unable to get back to her while he was being hunted. In his dreams he saw the creatures dragging her underground, but now they have the lode-seed they can go somewhere together where the ants will never find them. They’re about to leave when they hear Nyssa and Acheron returning. Waiting for the right moment, Yanesh leaps out at them and fires at the Menoptera, then he grabs Nyssa and drags her away. Nyssa pleads with him to treat Acheron‘s burns, but Yanesh insists it was his own fault for getting in the way. He tells Xanthe that he saw Nyssa with the monster ants earlier and she can show them how to ride the creatures and help them get away. Nyssa refuses to help, but Yanesh drags her outside. Weakly, Acheron calls after her…
The Doctor is furious with himself for taking Xanthe from the hive and for not spotting the answers that were staring him right in the face. Hedyla is exhausted and tells him she can’t run much further, but the Doctor says they need to get the two humans together quickly. They arrive at the Tower house and see the hole that’s been burned into the wall. They find Yanesh outside, holding Nyssa hostage and when she tells them about Acheron, Hedyla goes through into the laboratory to help her father. Nyssa has been forced to saddle Arbara and Yanesh orders her to climb onto the ant. Yanesh may have escaped from the Zarbi before, but Xanthe didn’t and the hive physically changed her because that’s what she was made to do - to change and adapt. Yanesh calls to Xanthe and she joins him, telling him they must go back to the hive and return the lode-seed. Nyssa realises the Doctor has worked out that the two humans aren’t really human at all. He reveals that they’re gene-synthetics from a seed ship carrying a nation of two. The Doctor explains that humanity decided the best way to spread out into space wasn’t through colonisation or invasion, but by sending out seed ships. Yanesh confirms that he and Xanthe were grown and sent to find dead worlds and barren places where humans can’t live. They would land there, adapt to the local circumstances and then wait for their biological programming to kick in, whereupon they would merge into one creature - one enormous cocoon - from which hundreds of children would hatch. Their offspring would be humans biologically adapted to complement and enrich the local ecology. The Doctor says they should never have ended up on Vortis as they must have been overwhelmed by the lifeforms that already lived here. Yanesh admits that he and Xanthe have only days left to live and he asks the Doctor to let them live those days in peace. Xanthe still insists that they return the lode-seed to the hive so that it can be healed and she reveals to Yanesh that she is the Speaker for Mother Life. Yanesh realises she is no longer the Xanthe he remembers and he lashes out at her. He grabs hold of Nyssa instead and drags her up onto Arbara, then they both ride away before the Doctor can react. Hedyla emerges from the Tower and tells the Doctor he needs to get inside quickly as the Zarbi are swarming towards them…
The Doctor checks on the injured Acheron and finds his friend in great pain. The Doctor blames himself for what’s happened but Acheron says the harm was done long before he arrived. The Zarbi swarm has moved on, but they’ve left a lot of damage behind them, including to the Tower itself which no longer looks structurally safe. Acheron admits that the Zarbi are known for not looking where they’re going. Hedyla returns from the lookout and reports that the Zarbi are following Yanesh and Nyssa, but keeping a safe distance behind them to avoid harming the lode-seed. Unfortunately they’re heading straight into the path of the Migration. The Zarbi navigate by the gravitational field of the lode-seed and it must have been a geological marker for generations, but now the Migration has been diverted. If the seed dies with Yanesh, the damage cannot be undone, but the Doctor can’t even help Nyssa unless he flies there and Acheron won’t have recovered enough to carry him for another few days. Acheron turns to his daughter and tells her it’s time she put her pet project to some use! Hedyla protests that it’s not been tested, but he reminds her they’ll never achieve anything without experimenting and there’s no time like the present.
Nyssa pleads with Yanesh to stop as he’s leading them straight into the Migration, but he tells her they must get as far away from the hive as they can. Nyssa asks him if this is how he really wants to die, but he’s too distraught at the thought that Xanthe has turned against him. He believes the Zarbi will tear him to pieces after he stole the seed from them, but Nyssa suggests they might actually be able to help him. Yanesh is determined to get off the planet, but then he screams out in agony and falls to the ground. Nyssa goes to check on him, but then Arbara races off as if startled by something. Yanesh thinks he sees a butterfly up above them, but then they hear the Doctor calling them to watch out. Guiding the glider as accurately as he can, the Doctor comes in to land, but crashes into a nearby tress and ends up bruised, but not too badly battered. He hopes Hedyla has a spare pare of wings as these ones aren’t going to be much use from now on. Yanesh is still alive, but it doesn’t look like he has much time left. Nyssa reminds the Doctor that the Zarbi Migration is heading their way and he turns to the hive ants and orders them to stop. He tells them he wants to talk to the Speaker for Mother Life and Xanthe emerges from the crowd and stands before him. The Doctor promises he won’t try to separate her from the hive again and says he only wants to help. She tells him the hive needs the lode-seed back and the Doctor shows her the dying body of Yanesh that contains the seed. He tells her she can’t have the seed as it’s dying with him, but he assures her the hive will remain unbroken because she herself has adapted to the point where she has a Zarbi mind. The synthetic part of her can no longer grow, but the Zarbi part can.
The Doctor produces Acheron’s Summoner and begins playing it. The effect on Xanthe is immediate and the Doctor tells Nyssa it’s activated her genetic programming and she’s responding to the Zarbi mating song. If he’s right, it should also activate the final phase of the life cycle. Yanesh and Xanthe call for each other, then Xanthe once again becomes the Speaker for Mother Life and the two humans begin to physically merge together, fusing into a cocoon. The Doctor reminds Nyssa that they have hundreds of adapted humans coded into their bodies. In this case, they’ll take after Xanthe and will be born as adapted Zarbi.
The migrating Zarbi arrive which means the Doctor will need to collect the cocoon and take it to safety before it’s crushed. Nyssa believes the hive Zarbi are moving in to fight the others, even though they’re hopelessly outnumbered and won’t have a chance, but the Doctor realises that‘s not their plan. These Zarbi are being fed by programming from the Speaker’s mind, which is now being influenced by the Summoner’s mating call. The Zarbi aren’t fighting at all, but engaging in quite a different form of contact. The Doctor suggests Nyssa avert her eyes and, in fact, it might be better all round if they beat an honourable retreat and let the Zarbi get on with it. This is rapidly turning into a massive Zarbi wedding and although he finds the marriage of opposites is somehow appropriate, he can‘t begin to imagine what their children are going to be like!
Sometime later, Acheron calls down to the Doctor as he flies gleefully around in the air. His recovery has been swift, no small thanks to Hedyla’s ministrations. He reports that the hive is returning to the Desolation and most of the Migration has gone with them, with the remaining Zarbi dispersing into the forest. He says he would have flown after them, but he can only stay in the air for a short while. The Doctor assures him his wings will heal in time and Hedyla plans to make other wings for herself. Acheron says he saw the wild Zarbi carrying something large, but he wasn’t close enough to see what it was. The Doctor guesses it must have been the synthetic cocoon, ready to give birth. Nyssa is concerned that it might cause trouble to have all these new humans running around, but the Doctor says that to all intents and purposes they’ll be fully integrated into the hive. Nature thrives on change and renewal, and now that Mother Life has both of them in abundance it‘s unlikely she‘ll need to lode-seed anymore. Hedyla admits that the Menoptera won’t like a whole new super-species on their door, but the Doctor says they’re just going to have to lump it. Acheron laughs and says he’s planning to go back to the city. He no longer wishes to spend his life out here alone, living like a hermit. The Doctor and Nyssa have shown him there’s still a lot he can do for Vortis and the Menoptera have been without his voice for too long. Nyssa is worried about Acheron’s enemies and their followers, but Hedyla assures her the miscreants responsible for attacking her have now been caught and she’s no longer afraid. She will stand by her father and face the future together. The Doctor wishes them luck, then Nyssa assumes he’ll want to head back to the TARDIS as they shouldn’t have any problems taking off now that the lode-seed has gone. Acheron says it will be a while before he and Hedyla return to the city and invites them to stay with them a little longer. The Doctor ponders for a moment. He always thought that if there were no panics or crises going on, then Vortis would be the perfect place for a holiday.