When Kanjuchi and Adiel arrive to one of the new chambers in the caves they check around but Kanjuchi finds something glowing like gold. He and Adiel get closer to take a nugget but it flips and rolls closer. Kanjuchi is astonished so he's too slow when the nugget-like blob stretches and reaches his hand. Kanjuchi tries to throw it away but the blob starts flowing all over his hand spreading all over his body. In pain, Kanjuchi screams and Adiel, horrified and unable to do anything for him, can only run away to look for help while Kanjuchi can't control his arms anymore.
The TARDIS materializes and when the Doctor comes out he finds crops all around him and finds it's really hot. He and Rose check the place and the Doctor discovers a sort of pollution due to weird alien static but the place looks very "earthish". While they're talking about it the Doctor notices that someone got behind the foliage around them and now gun barrels are being aimed to them.
The rifles are handled by two normal black humans who demand to know who the strangers are. The Doctor and Rose try to explain that they're just travellers but the two men are suspicious about the two white persons who apparently speak Arabic and just appeared from nowhere. Putting bits and pieces together the Doctor and Rose discover that they're actually on Earth, in Chad, on 11 April 2118. The two men's names are Solomon and Basel and they're still trying to figure out who the Doctor and Rose are when they hear Adiel’s screams.
They set off towards the sound followed by their new guests. When they break off the cover of the crop fields they see a huge mountain and a futuristic building hugging the ground beneath it. When Adiel sees them she tries to explain that something happened to Kanjuchi in the tunnels. The Doctor wants to go and see what's happened but Solomon disagrees and threatens him with his gun but the Doctor has no time to waste so he sets off to the direction Adiel came from, followed by Solomon while Rose and Basel try to calm down the girl.
The Doctor convinces Solomon that he could be of some help so they go through the underground tunnels and the Doctor realizes that the men are farmers who are growing genetically modified fungi inside a volcano and crops outside. Eventually they find Kanjuchi but it's too late: the man is dead and looks like a golden statue. The Doctor has a closer look but doesn't touch him. Solomon says that he need do inform Director of Development Fynn, who is in charge there and the Doctor agrees.
Basel leads Rose and Adiel into an empty common room where he gives Adiel a drink and once more tries to figure out who Rose and the Doctor are. Rose remains vague whilst Basel gives her some more details about the agricultural unit. After a while another man arrives and introduces himself as Edet Fynn, followed after some seconds by the Doctor and Solomon, back after discovering the dead Kanjuchi. The Doctor checks on Adiel and notices a peculiar necklace she's wearing, which is composed by glassy crystals often formed as the result of a meteor impact.
The others are more interested in discovering what happened to Kanjuchi and Fynn looks at Adiel's data-get readings but the data on the fungus crop looks like gibberish to him. The Doctor thinks that something's interfering with normal Earth geology but the others' attention gets focused on Nadif, another worker who arrives with a gold vulture in his hands. Solomon warns him to put it down scaring him into dropping it. Nadif explains that he found it in the west field while it was trying to fly away. After discovering this new mysterious living creature wrapped into that strange golden blob Fynn instructs Solomon to organise his teams to search if there are any more affected wildlife and Solomon and Nadif leave to start the search.
The Doctor talks with the workers about the possibility that the animals found some unknown way into the cave and Fynn decides that they must go see for themselves but he demands that Rose stays back with Adiel and Basel watches over her. Rose tries to comfort the still distressed Adiel when the golden vulture comes to life again and launches itself up from the floor flying straight for Rose's face. Rose tries to protect herself and Basel jumps on her to help but the vulture goes for the windows trying to break one of them and hits it until it smashes it and flies away. Basel and Rose run after it and see that it's flying to the volcano.
Fynn is checking the growth chambers with the Doctor following him. He's annoyed because the Doctor finally showed him an ID that proclaimed him to be from the Global Farming Standard Commission and it's really a bad time to have an inspection. The Doctor asks about the fungus farming and Fynn explains that they took a natural fungus that was already growing there and reengineered its DNA but the resulting fungus is poisonous. He states that it's just a matter of finding the right medium in which to grow the fungus. While they're talking, the Doctor and Fynn reach the place where Kanjuchi is standing, a statue of gold. The Doctor tries to analyse him with the data-get but the device is unable to recognize the material composition so the Doctor thinks that it's something alien and takes the sonic screwdriver out of his pocket to analyze the statue again. Suddenly Kanjuchi starts moving and hits the Doctor then stops moving but he has managed to take the sample he needed.
Rose finds the vulture but it vanishes again, apparently into a ledge in the foothills of the volcano. She and Basel go to the volcano and find what appears to be the vulture's nest and also a hole, apparently made from inside the mountain. A golden blob pops out of the hole and seeing it Rose and Basel take several steps backwards but the blob starts rolling towards them so they run away, the blob following them. Arriving at the makeshift billycane stairway Basel grabs one of the billycanes, creeps behind the arriving blob and brings the billycan down, spour first, on the animal, trapping it inside, then Rose stacks more canisters on top of it and around the sides. Hoping that the blob will stay trapped they go find the Doctor and Fynn.
The Doctor thinks that actually Kanjuchi is dead but something is controlling his body through the golden blob. Fynn wants to check the crops and sees that the fungi are covered by the golden blob as well. The Doctor thinks that there's an alien intelligence controlling the blob, which is targeting organic life. While they're talking the vulture arrives forcing them to move to a dead end.
Again the vulture attacks the Doctor, who tries to use the sonic screwdriver as a defensive weapon with success using ultrasounds. The Doctor thinks that the blob is converting living creatures to protect something but he and Fynn must leave to avoid more attacks.
Solomon is walking through the abandoned eastern caverns and reaches a dead end but he starts clearing rocks that open a hidden passage and at its end there's a man who wants food. Solomon explains that he must be careful otherwise someone will discover him while he's taking food, if so he won’t be able to bring more. The two argue for a while then the man goes away. Solomon goes back into the tunnels and reaches an area where there's a golden material just like the blob. He didn't report it because he didn't want to have people crawling so close to the place of his secret meetings. Solomon starts hammering the roof until rocks fall down, closing the chamber. He goes away and doesn't see Adiel watching him from the shadows.
Rose follows Basel into the entrance of the growth chambers, where bats are living up the ceilings but they discover that the animals are covered in golden blob. When they hear a weird noise in the darkness Rose calls for the Doctor and he answers that he's arriving with Fynn. Suddenly the bats go into a frenzy and the group is forced to run to the exit but the Doctor manages to capture one of the bats to study it. When they are safe the Doctor asks Fynn to send the staff away and Fynn agrees to send away the non-essential workers. Adiel arrives and asks about Kanjuchi so the Doctor has to tell her that he's dead and it's time to understand what's happened to him.
After studying the bat the Doctor concludes that the golden blob is a magma form that turns living creatures into sort of golems, and that the cause is due to something very old reacting to another factor related to the recent pollution picked by the Tardis. He wants to study the uncharted caves using the data-get after he increases its range and sensitivity. The first thing the Doctor gets is that Adiel's necklace isn't just composed of meteorites bits but that its composition is alien. Adiel explains that she got the necklace at a village nearby while on vacation. When the Doctor starts further studies on the necklace Adiel asks Fynn for a rest period and goes away but the Doctor is suspicious so he asks Rose to follow Adiel with discretion.
Rose reaches Adiel with the excuse of wanting a drink while she and Basel are talking in low voices but when they see her they switch to Kenga and start arguing about the situation, agreeing that it could be used to draw attention about the state of the environment but they have different ideas about other subjects. When Adiel says that she saw Solomon hide the golden panel Rose snaps so they realize that she understood everything and the fact that she understands an African dialect barely spoken outside Chad makes them suspicious. Rose tries to focus on the fact that the Doctor can help them and considering the situation they decide to go and check what exactly Solomon decided to hide.
The Doctor and Fynn start analysing the data they gathered which indicates that the necklace came from the same place as the golem-maker. The Doctor is sure that someone from space is waiting nearby but he needs more data so he asks Fynn for a ride around the volcano and while Solomon is driving the Doctor around the Volcano a huge flash of light dazzles him, forcing him to stop the pick-up. The Doctor tells him that the data-set picked up a massive energy surge from the flash but also from inside the volcano.
Rose is following Adiel and Basel when the man bumps into something then the same thing happens to Rose. Adiel confesses that she used roof props to keep them out of the way because soon someone's going to attack. She leaves them, ignoring their demands for an explanation. Basel tells Rose that neighbour villagers must have decided to come and get all the food they need and some of them are ruthless rebels. They're still talking when Rose sees a creature, clearly alien, scuttling across the red stone towards them.
Rose and Basel stare at the alien, wondering what it is. They think that at least the energy wall created by the roof prop can protect them but the alien disables it. Rose and Basel can only run into a dead end but the roof's caved in so they try to dig themselves out. They crawl into the hole and arrive into another cave full of skeletons and the alien is still behind them.
Fynn calls the main gate guards with the radio but can hear static only. In the distance he can hear a sound of breaking glass so he hurries towards the main gate but he finds the guards covered by the golden magma just like Kanjuchi. Fynn tries to call the Doctor and Solomon but the radio seems dead. He sees a golden blob coming towards him so he runs away towards the volcano.
Rose and Basel look for another escape way but the alien reaches them. In Arabic he introduces himself as Jaxamillian Faltato and gives them construction tools demanding that they break through a certain wall, threatening them with a very long grey tongue showing them how he can use it picking a skull and throwing it in a dark corner of the cave.
Adiel comes back from the caves and finds Fynn running towards them. He can barely whisper an explanation of what happened at the main gate and Adiel desperately calls anyone at the radio but with no answer.
Basel and Rose are tired from working. Basel checks the wall and finds the weakest spot so he and Rose can quickly finish the job and open their way into another cave where they find real gold. Faltato warns them to stay where they are and touch nothing then goes into the cave. Rose and Basel stare at what seem to be statues and paintings but then among the golden stuff the blob arrives and blocks the humans while Faltato stands still and watches. Rose asks him for help but he confesses that he wanted to use them as bait all along.
The Doctor arrives and, seeing the statues and the paintings, understands that they're Valnaxi art treasures; explaining that they were great artists caught in a devastating war. The cave is a vault where the blob protects the Valnaxi treasures and Faltato is a thief come to steal them. Suddenly the blob starts rolling towards the Doctor, who quickly moves to one side shouting to Solomon, who's hidden in the shadow. A haze of green light hits the blob, making it ripple like molten jelly. However, it's not dead and after a few seconds it starts moving towards Solomon. Faltato is checking the blob performance to conclude that the cave must be the last Valnaxi art warren. The Doctor and the humans run away from the blob and Basel is ahead of the group but outside the cave he soon finds Solomon facing a group of men with plasma guns aiming at him.
Adiel and Fynn run until they find Solomon's jeep. The keys are still in the ignition so they take it but very soon Kanjuchi emerges from the crops coming towards them.
The Doctor and Rose are still running from the blob, the Doctor explaining that the Valnaxi hid their treasures all over the galaxy. They see Solomon and Basel held at gunpoint by three men and they get caught too. The Doctor tries to warn them about the blob but one of the men threatens him with his gun and finding it close to his face the Doctor whips it away, distracting the other men so Solomon grabs hold of his gun barrel. The two start wrestling with the machine gun, which fires, hitting the ceiling and causing a terrible noise. The third man is distracted so the Doctor takes his gun and gives it to Solomon but a little earthquake starts, shaking the cave and the rebels run away. Right after then the blob gets close so the group is on the run again.
Fynn pushes on the accelerator and the jeep lunges forward. When they arrive at the entrance to the eastern network they find two men carrying a third one covered in gold. A fourth figure appears from the mouth of the tunnels, aiming a rifle at them and telling them to stop but Fynn floors the accelerator while the rebel opens fire.
The Doctor, Solomon, Basel and Rose run into a large, vaulted chamber full of abstract objects in stone and metal and try to barricade it, pushing a big canvas-like object to the opening, hoping that the blob creatures won't damage what they must protect but they can't cover it all so the golden Bats can slip in and attack Solomon.
Basel tries to help Solomon but there are too many Bats. The Doctor aims his sonic screwdriver at the Bats and activates it, causing them to fly away shrieking. The Doctor explains that there's enough Bat left in the creatures to make them sensitive to ultrasounds but the next time they won't be caught by surprise. The group tries another cave and find it filled with art as well. The Doctor says that they're like a safe within a safe with the blob protecting them: some alarm system woke the blob after detecting Faltato's starship and Fynn's excavating close to the safes.
However, Faltato is just waiting for them to open the safes and keep the blob busy for him. He decides that it's time to send a message to his sponsors in the starship hovering in orbit.
Fynn manages to drive the jeep out of firing range but he runs into two golden human figures and when he tries to run them over the jeep goes out of control, hitting a pile of metal canisters. The engine stops and Fynn can't turn it on again so he warns Adiel to get out but a stream of golden ants is waiting for them.
The Doctor leads his group to the growth chamber where Adiel saw Kanjuchi change and he realizes that the mushroom is not covered in gold anymore but the blob is still there and attacks them, hitting Solomon and sucking him back into a hole. The others try to go after him but they must avoid various golden animals coming after them.
Fynn's seat belt is stuck and he thinks it's over for him but the golden ants just swarm past them, followed by many other golden animals. Then the rebels arrive too and they stop for them, demanding that Fynn helps their comrade covered in gold but Fynn tells him that there's nothing that can be done for him. While they're arguing the rebel covered in gold attacks one of his former comrades and joins the golden animals. The rebel leader is still trying to figure out what's happened when he sees a vivid blue light pulsing in the sky.
The Doctor's group makes it out and they see all the golden creatures in a sort of parade, waiting. They notice the light in the sky and they're reached by Faltato. When the starship gets close the Doctor recognizes it as belonging to the Wurms, the species that wiped out the Valnaxi. When the starship lands and dozens of Wurms came out the golden creatures attack them.
Adiel, Fynn and Guwe, the rebel leader, watch the worm-like aliens through a window of the labourers' block and the battle with the golden creatures. The invaders are using some kind of mud guns that can destroy a golden creature. Guwe thinks that Fynn drugged him so he hits the director.
Outside, Faltato explains that the Wurm weapons are standard for taking care of the guardians and that after the Wurms take the Valnaxi vault his reputation will get a boost. Two tongues lash out of his mouth, slapping themselves around the Doctor's neck and Basel's waist. Rose can only run away.
Guwe is still trying to understand what's happening but Adiel can't explain it and now Fynn is unconscious. Guwe wants to kill Adiel but before he can shoot the door is kicked open, Rose dives in and knocks Adiel down while a shot from a Wurm hits Guwe, killing him. Rose explains to Adiel what happened. Meanwhile, Fynn wakes up. They all can only stare at a Wurm when the alien reaches them and declares them prisoners of war.
Faltato and his prisoners are reached by a group of Wurms, lead by their King Ottok and his assistant Knight-Major Korr. Faltato tells them that he found the deactivation plaque so the Wurms can destroy all the Valnaxi relics and then the whole Earth.
The Wurms use their prisoners as human shields but the golden creatures don't attack them so the Wurms suspect that the humans are allied to the Valnaxi creatures and go report to the King.
King Ottok and Korr receive the news and demand an explanation but no-one can give any. The Doctor says that he can help the Wurms to get the Valnaxi art while Faltato has only limited information; he also says that he needs memory wafers for the data-get to work properly and when he sees on a monitor Fynn, Adiel and Rose he adds that Fynn is the director so he can convince the staff people still hidden to surrender and be used as a protection from the golden creatures.
After being brought to a prison area, Fynn and Adiel start arguing about old local stories of ethnic fights and Fynn confesses that he bought human corpses to feed the fungi. While Rose is harshly commenting Fynn's behaviour the Doctor is brought to the area and they're all brought to the caves.
The Wurms are advancing using the Doctor and the humans as shields and the Doctor is told that Solomon brought the roof down on what could be the activation panel mentioned by Faltato. A guardian blob tries to attack but the Wurms shoot it. Amidst all this it manages to touch Rose's feet. The Doctor tries to pull her clear but it's too late and the golden blob covers her.
The golden Rose and the guardian run away and the Doctor takes advantage of the chaos to run away, followed by Fynn, to the lab unit. The Doctor wants to create a serum that gets rid of the golden magma like the fungi did probably thanks to their cellular features, then let the blob infect him hoping that the serum will prevent it from taking full control and possibly find some Wurm's weakness to use to defeat the alien invaders and use the serum on Rose to take her back before her body chemistry is too far gone.
Basel and Adiel are taken by Faltato to the cave where the activation plaque was buried by Solomon but after examining it Faltato realizes that it's a fake made just to fool him or anyone in search for the Valnaxi treasures.
After experimenting for a while the Doctor finds the right way to produce the serum he's looking but Korr finds them and demands that they go back to him. The Doctor threatens to destroy the memory wafers the Wurms need to achieve their goal but Korr thinks that the Doctor is building a weapon so he doesn't step back. Suddenly an explosion blows up the lab door and hits Korr: Fynn caused the explosion but he's badly wounded and soon dies. Adiel's necklace was crushed as well and the magma traces contained in the stones start moving towards the Doctor so he injects himself with the serum and lets the blob reach him. The serum reaction is painful but he's not controlled so he goes to find the blob guardians and Rose and soon finds them and lets them try to take him.
Adiel and Basel are loading some canvases in a transporter but Faltato is unhappy because he examined them and they're not the most ancient and famous Valnaxi treasures he expected to find so he senses that there's something wrong.
King Ottok is watching the Valnaxi treasures being taken when Korr arrives, badly wounded, and tells his king what happened. After promising Korr that he'll have another fight King Ottok goes to fix the data-get to have the information he needs about the Valnaxi defence.
The Doctor tries to stay lucid and uses the data-get to collect more information and discovers that whatever is controlling the blob is hidden deep in the ground. Checking around he discovers a teleport and goes down where he demands to have Rose back. From a doorway two golden figures emerge: one is what remains of Solomon and the other is Rose but her body is distorted, indicating that it's too late for her to be saved.
A voice in the Doctor's head demands him to surrender and be part of the fight against the Wurms but the Doctor doesn't trust the mysterious controller so he refuses. The golems surfaces start cracking and two humanoid figures emerge, carrying faces similar to Solomon's and Rose's but without the original details. The male golem explains that they are the future of the Valnaxi race and they must stop the Doctor's mental disruption before the Wurms destroy them all.
Faltato is still looking for the last Valnaxi treasures when he's contacted by King Ottok, who announces his victory against the guardians and that he'll be down with him shortly. Adiel and Basel understand that they're not needed anymore so they try to attack Faltato but his pincers and tongues are too strong for them and they have to face Faltato's rage.
The new golems attack the Doctor but they're weak so the Doctor can easily break their grip and once again demand that the real Rose is set free and Solomon as well but the golems object that they don't know how to do it so the Doctor uses his serum then surrenders to the controller.
Faltato's retaliation is interrupted by King Ottok, who's impatient to take possession of Valnaxi's greatest treasure but Faltato must admit that he hasn't found them causing the king's anger. Faltato tries to calm him promising he'll have the treasures found very soon but King Ottok doesn't believe him anymore and orders to have him executed. Faltato grabs Adiel and Basel and holds them before himself, though they'll not shield him from the Wurms.
The Doctor realizes that the magma gave up on him thanks to the serum so he looks around in the cave and eventually finds the controller, who turns out to be the disembodied intelligence of the Council of Valnax, that hid there to set a trap for the Wurms, knowing that they would come for their treasures. They also wanted to create a new template for themselves to be back with a new form but they didn't expect it to be human, rather they thought it would be the Wurms form so they could walk between their ancient enemies without being recognized.
King Ottok orders his soldier to kill Faltato and his human shields but two golems appear from nowhere.
The Doctor is still talking to the controller when he realizes that the serum effect expired and the controller knows it too because the golem surges towards him.
When Adiel opens her eyes she sees Solomon close to her and the Wurms shooting golden animals. She also notices that Rose is there too and she's leading Basel to shelter while Faltato is already gone so she follows them. Solomon and Rose explain what happened but when the Wurms gunfire stops Faltato grabs Rose and goes back to them.
The Doctor is trying to avoid the golems while asking about their plans and the male golem explains that after the imprinting of human DNA on the Valnaxi fake plaque the controller realized that human reproduction is inefficient so they need a way to assimilate human beings and replace their minds with Valnaxi ones. To escape the golems the Doctor grabs one of the conduits snaking up to the machinery set high in the walls and hauls himself up and out of reach.
Faltato brings Rose to King Ottok and reveals him the location of the Valnaxi teleport and the secret cave. Ottok finds the energy signature and orders Korr to get rid of Faltato, who quickly ducks to hide from the Wurm's fire while Ottok leads his troops to the Valnaxi controller.
Faltato wraps six tongues around stalactites while using his pincers to hold on to Basel and when Korr is close enough he guides himself through the air so that Basel can hit Korr in the face throwing the Wurm in the air. Faltato and the humans run away while Korr lays still.
Still clinging on, the Doctor witnesses the new battle between the Wurms and the Valnaxi golems but soon the Wurms start shooting at him as well and after a while all their shots cause a hole in the pipe and magma starts flowing out, engulfing the Wurms. King Ottok keeps on firing but bits of blob hit him too and eventually he's killed. Amidst the chaos Rose appears and gives the sonic screwdriver back to the Doctor, so now he can operate the Valnaxi control panel. He turns on the ancient starship's engine, triggering the take off sequence. He urges everyone to get out but the teleport doesn't work anymore, damaged in the fighting.
A Wurm survived to the battle wants to kill them but the Doctor uses his sonic screwdriver to turn his battle implants to the max putting him out. Rose tells him that the Wurms used their gear to open the teleport so the Doctor activates it and they go through, including the two new Valnaxi golems. The Doctor wants to help Korr but the Valnaxis complain and the Doctor warns them that with such an attitude he won't let them stay on Earth so they get convinced to carry Korr to his starship while the caves start shaking.
Faltato realizes that a starship is taking off out from the volcano so he, Adiel and Basel run to the Wurms starship, hoping to be protected by its force-field.
The Doctor, Rose and the Valnaxis carrying Korr are close to the Wurms starship when the Valnaxi starship takes off and the volcano explodes. They expect to be hit by all the debris but the force-field protects them and from a hatch in the Wurms starship Adiel greets them. She looks at the Valnaxis and asks the Doctor about them but he answers that there's no-one with them, glancing at the golems staring deep into their golden eyes. The Doctor and Rose carry Korr into his starship and when they're done the Valnaxis are gone.
Korr and Faltato take off but under his warrior code Korr owes Faltato his life and his loyalty. With no trace of aliens anymore officially everything was caused by the volcano's eruption. When the workers come back they can clean all the mud left by the Wurms and it turns out that anything can grow in it so Adiel, who's now the acting Director, is planning to use it as a resource but Solomon won't be there because he decides to resign and go back to his old village and help the people there.
While the Doctor and Rose leave for their next destination, on the edges of the desert the two Valnaxis are planning their future on Earth, where they want to start making art again.