Trooper Delong wakes up with a scream after yet another terrifying nightmare. The other soldiers laugh when Trooper Salway taunts him and suggests he’s wet the bed. The abuse becomes even more personal when Salway makes racist comments about Delong’s ‘indig’ origins, but before things can escalate further, the soldiers are interrupted by a ship-wide announcement from Admiral Farroll. She tells them the fleet is about to go into orbit around Carista 7 and preparations are underway for a full-scale attack. As the squad gets ready to be dropped down to the planet surface, they cheer at the prospect of getting some “payback” against the bugs.

The TARDIS materialises and Lucie is devastated to see that they’ve landed in yet another mucky spaceship. She recalls the Doctor promising that the next time this happened they’d leave straight away, but the Doctor’s attention has already been taken by a series of pods filled with guns. They hear an announcement declaring ’general quarters’ and suddenly the door at the end of the corridor opens and a squad of troopers spot them. They immediately open fire…

As the ship goes into orbit, Admiral Farroll notices a strange reading on their instruments, but her first officer Winslet dismisses it as the effect of solar radiation on the scanners. To their surprise, the two troopers Salway and Delong arrive on the bridge with two strangers who were found near the launch bay. Farroll starts to question them, but the ship is suddenly rocked by an explosion and Winslet reports an impact on the starboard hull. They soon discover that every ship in the fleet has come under attack at the same time. Winslet says the repair drones are unable to cope with the damage and they’re about to lose hull cohesion. Farroll realises she has no choice but to abandon ship. Just then, the metal walls to the bridge are ripped apart and several huge Wirrn swarm inside. The Doctor yells out a warning to everyone not to let the creatures touch them, but it’s too late for Winslet and they hear the first officer scream.

Salway and Delong open fire on the Wirrn while the rest of the bridge crew race out into the corridor. The Doctor and Lucie head off in the direction of the launch bay but soon become lost in the sprawling corridors. As the ship only has moments left, they climb inside spacesuits for protection. They’re blasted by a terrific wind which tells them they’re about to be blown out into space, so the Doctor urges Lucie to hold on. Moments later the entire section is de-pressurised, just as more Wirrn start to crawl inside the ship. The Doctor tells Lucie this is a full-scale war with both the humans and the Wirrn locked into a battle for survival. Lucie thinks the ’bugs’ can’t possibly win against heavily-armed spaceships, but then she sees how easily the Wirrn can peel back the bulkheads. To her horror, she also sees the TARDIS drifting away from the ship after having fallen out into space. The Wirrn are starting to get really close so the Doctor shows Lucie how to activate her spacesuit jet-pack. Then they literally fly down the corridor passed the approaching Wirrn and out into open space…

It’s not long before the fuel cell in Lucie’s emergency jet-pack expires and she finds herself drifting in space, with the explosions from the destroyed fleet still rumbling away in the distance. Panicking, she calls out for the Doctor and a few moments later he joins her and shows her how to switch on the communicator inside the spacesuit. There’s no sign of the TARDIS anywhere and Lucie’s worried it could have drifted miles away by now. They can see people falling through space and being sliced apart by the Wirrn, then they start picking up distress signals from the other ships. It’s a massacre and there’s nothing they can do to help. Although some of the landing pods are landing safely on the planet below, the Doctor suspects the slaughter will simply continue down there. Fortunately the TARDIS has gone into a spiral, circling around the planet and it soon reappears on the horizon. Using the last of the fuel from his own jet-pack, the Doctor guides Lucie towards their ship. Lucie asks whether there’s anything they can do to help the others, but the Doctor says this is one of the human race’s bloodiest periods. Although he’s never been here before, he’s heard of the GalSec colonists. They forged out into space after the evacuation of Earth, but there were many battles to fight for survival. He says trying to make a difference here would be like standing up in the middle of the Somme and calling for a ceasefire.

Unfortunately, before the Doctor and Lucie can reach the TARDIS, they’re “rescued” by a passing escape pod. They emerge from the airlock and find themselves confronted by Troopers Salway and Delong, who’ve been searching the area for other survivors. Their situation is desperate and they have with them the badly injured Admiral Farroll, who was attacked by a Wirrn shortly before they escaped. Salway starts to brag about how he blew the huge bug away, but Delong remembers the stranger was described as a ’doctor’ and asks him to look after Farroll. Their priority changes when the escape pod’s auto-stabiliser blows, which means they won’t hit the planet’s atmosphere at the right angle. Knowing they’ll burn up unless he does something to help, the Doctor insists on being shown the pod’s controls. Despite Salway’s angry protests, the Doctor takes the helm and tells everyone to strap in as they’re in for a bumpy ride.

Eventually the escape pod enters the atmosphere and comes crashing down onto the planet surface. Not far away, the Wirrn Queen hears their arrival and orders her foot soldiers to go out and find any survivors.

The Doctor and the others emerge from the pod and realise it won’t be long before dawn. Salway grudgingly congratulates him for getting them all down in one piece, but he continues making derogatory remarks about them being ‘indigs’ and Lucie demands to know what he means. Delong explains that he’s referring to the original indigenous settlers who came out here centuries before GalSec got organised. The discussion has to stop because they’re in danger from the debris of falling spaceships. There were 68 ships in Admiral Farroll’s fleet, but it doesn’t look like many of them survived the battle. They hear the sound of Wirrn approaching and Salway orders Delong to help him fight them off. Lucie points out that two guns are unlikely to stop many of the bugs and suggests they get inside the escape pod.

Hundreds of Wirrn appear in the distance and quickly surround the pod. Delong suggests they use the gun turret on the roof, but they know it won’t help for long. As the Wirrn climb on top of the pod, the Doctor starts diverting power from the battery so he can electrify the hull. He tells the others to rip away all the seat padding and build a pile in the middle of the deck to insulate them from the charge. As soon as everyone’s safe, the Doctor activates the power and the attacking Wirrn are electrocuted. The other Wirrn start to retreat, but unfortunately the Doctor had to use nearly all their power and it’s likely the creatures will be gone for long.

The Wirrn regroup not far away and the Wirrn Queen realises there is a “wise” one amongst the group of human intruders. It can sense the Doctor’s burning intelligence and warns her people he could be a threat.

Sometime later, dawn rises on the planet and the group inside the escape pod can see for the first time that the landscape is littered with dead bodies and wreckage, stretching right back to the horizon. Salway angrily spits out abuse towards the Wirrn and says they’re going to pay for what they’ve done, but Lucie points out there are just as many dead Wirrn out there as there are humans. The Doctor asks if they’ve tried to negotiate with their enemy, but Delong says you can’t reason with animals. Salway confirms that the Wirrn arrived all over GalSec territory and started killing people. Lucie questions their right to this territory, but Delong dismisses the Wirrn as nothing more than a plague. He says they stripped whole planets clean of crops and cattle, leaving the people starving. The Doctor recalls that the Wirrn use host bodies for their hatchings and points out that although that may sound disgusting, it’s a pattern repeated all over the Universe. He adds that the Wirrn inherit the race memory and intelligence of their hosts, which means that if the Wirrn Queen here was born from cattle, she’ll have severely limited brain power.

The Doctor and Lucie check on the injured Admiral Farroll and realise she needs to get to a hospital fast. There’s not much left in the first-aid box, but Lucie applies some self-sealing bandages to the Admiral’s wounds. The Doctor asks Farroll about the Wirrn that attacked her, but she’s too weak to reply so he administers some pain relief. He realises that if they have any chance here, they must find more power and team up with anyone else who survived. Ideally, they’ll need a working escape pod with a transmitter. It means they’re going to have to go outside and look around, but Farroll clearly can‘t be moved.

An argument breaks out when the Doctor makes his suggestion to the others, but Salway soon finds himself alone in opposition to the plan. He starts to pick on Delong again, and the young trooper raises his weapon in a threatening gesture. This just makes Salway taunt him even more, but the Doctor warns the youngster not to rise to the bait. Delong orders everyone to be quiet and fires off a couple of laser bolts into the ceiling to emphasise his point. He then declares that as he’s the trooper with the most service credits to his name, he is the senior officer now. He agrees to go along with the plan - Salway and Lucie will stay here with Farroll while he and the Doctor go outside to look for survivors and supplies. As he leaves, the Doctor warns Lucie they only have enough power in the battery for one more shot, so if the Wirrn come back she must chose her moment well.

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.