Left behind to guard the escape pod, Lucie tries to make casual conversation with Trooper Salway. She comments on the way he treats his colleague Delong and accuses him of being racist. He arms his weapon and points it at her, but she points out that he needs to stay calm as he’ll be no use to her if he freaks out the next time they’re attacked. Salway asks her about her relationship with the Doctor and tells her she should be careful that he doesn’t let her down. Lucie can’t shake off the feeling that they’re being watched, so she tells Salway to stay alert.
The Doctor and Delong cautiously move away from the pod and explore the landscape. Delong spots something ahead which seems to be catching the light from the sun and they head off towards it. Delong realises that even if they manage to send off a call for help, it’s going to be a very long time before help arrives if the massacre of the fleet is anything to go by. The Doctor asks him about the indigs and Delong says the term is used by the GalSec colonists to describe anyone they don’t like. Delong works alongside the GalSec people and regards himself as one of them, but as soon as someone gets to hear about his origins, the abuse starts all over again. His grand-father was one of the last colony bosses during the time when the people lived like peasants and farmers. It was a tough primitive existence, but they never had any trouble with the Wirrn then. They suddenly stop when they see a group of Wirrn attacking some of the human survivors. Delong wants to help them, but the Doctor points out that there’s nothing he can do and he’ll only end up getting himself killed too. The battle is long and fierce, and Delong becomes despondent, realising they’ll never defeat the Wirrn. No matter what they do, the creatures will just keep coming back…
Outside the pod, Lucie and Salway are horrified when they see hundreds more Wirrn approaching. Lucie races back inside and prepares to close the doors behind them, but Salway is rooted to the spot, firing round after round at the advancing creatures. Eventually he’s forced to give up and joins Lucie inside, then he starts searching for the electrical cable that the Doctor used earlier. Lucie reminds him there’s hardly any power left and suggests they wait until the Wirrn are right on top of them. They hear the creatures climbing over the hull of the pod and Salway begins to panic, screaming to Lucie that they’re breaking in. Lucie calmly waits for the right moment, then she activates the power. They hear the Wirrn screaming in pain, and then shortly after it all goes quiet again. The Wirrn have gone, but so has the last of their power.
Farroll cries out the name of the Wirrn, so Lucie checks on her condition. Salway is reluctant to change the Admiral’s bandages and is happy to leave that to Lucie. They’re both shocked when they see the extent of Farroll’s injuries and all Lucie can do is give her something to help with the pain, but Farroll screams out in agony and begs them to kill her. Lucie discovers Farroll’s wounds are starting to turn septic, but Salway is more worried that the Admiral’s cries are likely to attract any nearby Wirrn. Lucie rebukes him for his lack of compassion and he becomes angry with her again.
The battle between the Wirrn and the other human survivors is over. There’s no sign of any movement and the Doctor thinks both sides have wiped each other out, but they soon come across one remaining Wirrn and one remaining human. Despite the Doctor’s protests, Delong instantly opens fire and kills them both. The Doctor turns angrily on the young trooper, but Delong insists his actions were necessary. He shows the Doctor that the Wirrn had bound the human to a kind of sacrificial altar. He says this is what the Wirrn have always done, even as far back as his grandfather’s day. At that time the indigs were little more than savages and every season, when dawn arrived, they would sacrifice one of their own people to the Wirrn. Delong remembers his grandfather was proud of their actions and that he believed it was the way of nature. The Doctor realises Delong’s grandfather was actually making a lot of sense and it’s the reason why the indigs never had to fight the Wirrn.
Farroll’s wounds have turned green and when it starts spreading, Lucie suspects the Wirrn have infected her with some sort of poison. She remembers the Doctor telling her the Wirrn use other races as their gestation hosts and they suspect the Wirrn that attacked Farroll must have laid an egg inside her. The Admiral starts to mumble about her species surviving and when they realise she’s talking as if she herself is a Wirrn, Salway charges his weapon and prepares to kill her. Lucie insists they wait for the Doctor to return, but Salway laughs and says the Doctor’s probably dead already. The two of them struggle over the weapon and Lucie is shot in the arm. Salway is about to kill Farroll when the hull of the escape pod is ripped apart and Salway is dragged screaming from the ship. Several Wirrn crawl inside and although Lucie tries to convince them their war with the humans has nothing to do with her, her words have no effect on them. Just then, they hear a sound that seems to be familiar to the Wirrn and, to Lucie’s surprise, the creatures turn and leave the pod. Farroll weakly turns to Lucie and thanks her for saving her from the “other human”.
The sound that the Wirrn heard is the Doctor striking a metal structure in exactly the same way that Delong’s ancestors used to when summoning the creatures to a sacrifice. He’s confident the Wirrn will have retained the sound in their race memory and will be attracted to their location. Delong wonders if the Doctor is planning to sacrifice him to save his own neck, but the Doctor assures him that’s not what he has in mind. It’s starting to get dark again and it’s not long before they see the Wirrn swarm approaching, dragging the shattered escape pod along with them.
Delong raises his weapon, determined that if he’s going to die, he’s going to take a good few of the Wirrn along with him. The Doctor advises him to take the wise course of action, just as his grandfather did all those years ago. The Doctor says the GalSec colonies were heading for extinction and couldn’t possibly win their war against the Wirrn. He believes the actions of people like Delong’s grandfather saved their entire colony, but he’s also convinced there was more to it than just a simple sacrifice. He has an idea what was really going on and wants to put it to the test…
The Doctor calls out to the Wirrn and tells them this is the way things used to be, the way of nature. The approaching creatures come to a halt and the Doctor tells Delong they recognise his words as the ones used during the ceremony all those years ago. The creatures communicate amongst themselves and then move aside to allow the huge Wirrn Queen to step forward. The elderly Queen speaks to the Doctor and Delong and reminds them that the Wirrn absorb the intelligence and memories from their host species. The Doctor explains that the Wirrn young gestate inside other species and although the host dies, their intelligence lives on. He believes Delong’s grandfather knew that and realised that as long as the Wirrn Queen had the intelligence of a human, they’d know to live in peaceful co-existence with the colonists. The Queen confirms this and says she is the last of the original Queens. She is close to death and lost control over her swarm when the younger Queens were gestated within senseless herbivores who had no intelligence. The Queen is aware that Lucie fought to protect the new Queen inside the pod and the Doctor realises she must be referring to Admiral Farroll.
At the Doctor’s request, the Wirrn allow Lucie to emerge from the escape pod. She’s surprised when the Queen speaks to them, but the Doctor explains that Wirrn can absorb the ability to understand the human language. When Lucie tells them that Salway finally went mad and shot her before being killed by the Wirrn, the Doctor congratulates her for standing up to him when he threatened to shoot the Admiral. The Wirrn are also grateful to Lucie and tell her they must all wait until dawn rises…
Later that night, when everyone else is sleeping, the Doctor hears Farroll calling out and goes to check on her inside the pod. She tells him she can feel a great blackness rushing in and asks him what’s happening to her. The Doctor tries to calm her down and assures her she’ll be at peace soon.
Hours later, Lucie wakes up and finds herself sitting on a sacrificial altar, surrounded by giant insects. Delong tells them he’s saved enough bullets for each of them in case the bugs decide to attack. The Doctor checks Lucie’s arm and tells her she’ll need to have it checked properly when they get back to the TARDIS. Fortunately he’s found their ship, but it’s still quite a distance away and it’ll take them at least an hour to walk there. As the sun starts to rise, they hear the Wirrn approaching with Admiral Farroll. The Doctor tells Delong to stay calm. He’s sure the Wirrn don’t intend to hurt them, but if they feel threatened then the old Queen will probably be too weak to stop the less intelligent Wirrn from attacking. The Queen confirms this and tells the Doctor her time is coming to an end.
Farroll speaks again about the great blackness and Lucie realises she’s talking about space. The Doctor says the Admiral is getting in touch with the Wirrn race memory but he hopes she’ll still be able to retain enough of her human side. Moments later, the metamorphosis is complete and Lucie is horrified when she sees Farroll’s features have been completely transformed into that of the new Wirrn Queen. The old Queen knows her work here is done and she passes away.
Lucie wonder what the new Queen has planned for her regime and suggests she considers benefit reforms, tax cuts and mercy to all humans. Delong is sickened by what’s happened, but the Doctor points out that it’s not a question of right or wrong and that he needs to broaden his personal horizons if he wants to survive and have peace. Delong refuses to listen and opens fire with his weapon. The Doctor tells him that the human race’s refusal to accept that this is nature’s way is what led to the endless war between them and the Wirrn in the first place. The new Queen repeats back Delong’s grandfather’s words to demonstrate that they’ve been passed down over generations and have become part of the Wirrn’s own race memory too. Delong starts to question his own disgust and realises that his ancestors were right to do what they did in order to survive. However, he also knows that the GalSec colonists will never agree to continue the tradition of sacrifice.
The Doctor poses this question to the new Queen and Lucie asks how they can hope to avoid another bloodbath when more colonists start to arrive. The Queen doesn’t have an answer, but she knows that it’s not the Doctor and Lucie’s fight, so she advises them to leave. Neither of them feel comfortable about just leaving them to it but the Doctor says the only thing he can be sure about now is that it’s nature’s way to try to survive. He offers to take Delong with them, but the young trooper prefers to stay. He says his people once worked out a way to live in peace with the Wirrn and maybe if he and the Queen work together, they can come up with an alternative way. The Queen agrees to help them find a solution that will lead to their mutual survival, so the Doctor and Lucie wish them all luck.
Back in the TARDIS, the Doctor treats the injuries to Lucie’s arm as they watch the sun rising over the crest of the planet on the scanner. Dawn over Carista 7 is undoubtedly one of the most beautiful sights they’ve ever seen. Just then, Lucie spots the Wirrn swarm leaving the planet and heading out into the great blackness. Perhaps Delong was able to convince them to leave before the human reinforcements arrived, or perhaps it was down to their new wise Queen. The Doctor sets the controls and the TARDIS dematerialises…