Earth is at war with the Ten-Strong, a cadre of Schirr terrorists who have been using the dark magics of the Morphiean race to attack the Earth Empire which annexed their homeworld. Only the best warriors get into Earth’s Anti-Terror Elite after exhaustive training, and on every training mission they wear neural net websets to record their experiences and impressions for later review. Admiral Nadina Haunt is now preparing to take a squad of trainees on their last exercise. Ten soldiers, including Haunt and her adjutant Shel, will be sent to a location pre-determined by the Pentagon Central computers, to try to destroy two Kill-Droids programmed to kill them. But some of the trainees have secrets; in particular, Matthew Shade, who rejected the power and privileges granted to the natural Earthborn and joined the ATE to give something back to the Empire. His ex-lover, Gisel Denni, doesn’t believe he’s really turned his back on the Earth -- but even she doesn’t know the true story behind the shrapnel which remains buried in his face, scarring him for life.

The trainees enter suspended animation and awaken near their target, a small planetoid near the edge of the Morphiean Quadrant. Shel releases the Kill-Droids into the training grounds, and the team then enters the tunnels and splits up, hunting the killer robots through a complex which seems to have been designed to resemble Schirr architecture. The complex is dotted with statues of cherubs, and filled with phosphorescent weed and bioluminescent fleas which make it impossible to track the other team members by their life signs. Denni teams up with the crass Joiks, but privately suggests to him that Haunt shouldn’t be in charge of this mission, as she has a history of irrational hatred towards the Schirr. Joiks waits until they’re further on in the tunnels, in the dark, to make his move; for if Denni wants him to act against Haunt, then he wants her to do something for him in return.

The TARDIS materializes in an airless chamber, and the Doctor, Ben and Polly don spacesuits and emerge to explore. They find themselves in an alien control room, set up like a shrine, and are horrified to find ten dead alien bodies -- one slumped in a chair in a pool of dried blood, and the other nine frozen in a force field like a cube of glass. As the chamber begins to fill with light and air, Polly suffers from a moment of disorientation and recovers to find that she has somehow run out of the room and is lost in the tunnels. Trying to find her way back to her friends, she finds a porthole looking out into space, the stars twinkling beyond like diamonds. She then spots a hypnotic blue light in the distance, and is drawn towards it, feeling that she’s seeing some kind of countdown. However, she is forced to retreat when she is attacked by one of the Kill-Droids.

Haunt and Shel find their way to the control chamber, where Haunt is shocked to see the Doctor and Ben and nearly shoots them both before Shel stops her. The Doctor points out the alien bodies, and Shel is taken aback by the prison brands seared into their flesh. These are the bodies of the Ten-Strong, the terrorist leader deCaster and his followers; the body outside the cube is deCaster’s second-in-command, Pallemar. Apparently, the most wanted terrorists in the Earth Empire are lying here dead in the squad’s training grounds. The Doctor tries to get back into the TARDIS, only to find that some force prevents him from opening the door. Haunt, still suspicious, orders Mel “Frog” Narda -- whose face and vocal cords were grievously damaged in childhood -- to take the Doctor and Ben back to the soldiers’ ship for questioning.

Tovel and Shade encounter Polly, and fight off the Kill-Droid pursuing her. In the process, they bring down the tunnel, blocking the way through to the cavern of blue light. The complex is then shaken by tremors, as if the rockfall has caused the unstable tunnels to collapse, and Frog, the Doctor and Ben find the way back to the ship blocked. As the tremors continue, Joiks arrives in a panic, claiming that something came out of the darkness and took Denni. Polly then realizes that the tremors resemble the shaking of the TARDIS as it dematerializes, and her suspicions are confirmed when klaxons sound from the control chamber. Tovel, a trained pilot, studies the controls in the chamber and confirms that this complex has broken away from the rest of the planetoid and is now in flight. Polly then notices that one of the Schirr bodies has vanished from within the glass cube; perhaps it was disintegrated by vibrations through the force field, or by one of the Kill-Droids. It can’t have moved by itself, for a scan confirms that the bodies show no life signs. But one thing is certain; this complex only became active when the soldiers arrived, which suggests that it is a trap of some kind.

The Doctor and his companions learn that deCaster and his followers objected to their planet being “repatriated” by the Earth Empire, and took advantage of their species’ ancient ties to the Morphiean Quadrant to steal knowledge of the Morphieans’ magics and use them as weapons of terror. The Morphieans are also attacking the Earth Empire, in retaliation for the theft of their secrets -- for the Schirr are technically a part of the Empire and the Morphieans seem to draw no distinction between corporeal life forms. This explains the political backdrop, but what happened in this complex remains unclear. Was there a power struggle between deCaster and Pallemar? In any case, the complex is being steered by a primed crystal navigational matrix, and the crystals have been removed; the Doctor and Tovel can work out where they’re going, but until the nav-crystals are found they can’t change course.

Haunt decides to concentrate on Denni’s disappearance, and despite the Doctor’s objections, she orders everybody to go out into the tunnels to look for her. Polly teams up with Shade and Lindey, fascinated and horrified by the injuries on Shade’s face. He tells her that they are the result of his shielding a group of innocent children from a fragmentation grenade; he could easily get his face repaired, but he’s chosen to wear the scars as a reminder. However, Lindey privately informs him that she knows the true story, and when they get out of this she intends to blackmail him to use his Earth connections on her behalf. She won’t get the chance, for in a particularly dark tunnel something snatches her away from them. Shade and Polly are unable to find her, but as Shade reports to Haunt, Polly notices that he’s picked up Lindey’s personal palmscreen and hidden it in his combat suit.

Ben, Tova and Roba are attacked by a Kill-Droid, and Ben wins the soldiers’ respect by helping them take it out. Shel, the Doctor and Polly are attacked by the other Kill-Droid, and Shel is injured before the others arrive to help destroy it. He seems disoriented by the injury, but insists upon treating it himself. The Doctor and Creben examine the Kill-Droids’ weapons and discover that the disintegrators have not been fired, which means that the Kill-Droids are not responsible for the disappearances of Denni, Lindey and the Schirr body. They return to the complex to ponder what they know, passing more cherub statues as they go, and arrive to find that a second Schirr body has vanished -- and that their projected destination is the heart of the Morphiean Quadrant.

Haunt realizes that Shade is in pain, and he is forced to admit that his face feels as though it’s bursting open. Frog prepares force mattresses for him and Shel, small malleable fields which expand to full size out of small capsules, while the Doctor spins theories about what’s happening. Are the Morphieans drawing the Schirr towards them to exact revenge, and did the Ten-Strong kill themselves upon seeing what was coming? The Doctor tries to study the controls to see what else he can learn, but Shel, who has been growing increasingly disoriented, pulls a gun on him. When Frog tries to shoot the gun out of his hand, Shel is revealed to be a robot, and he shoots the console which the Doctor was trying to examine and flees into the tunnels. This training ground was supposedly selected at random by the Pent-Cent computers, based on an analysis of the soldiers participating in the exercise -- but it would appear that somebody important altered the programming to send them here and sent Shel in to set up the trap.

Haunt sets off to fetch the others, while the Doctor and Polly tend to Shade, who is in great pain as the dead tissue in his face is forced out through his skin’s pores. The Doctor gives him a painkiller -- but in the few moments that his back is turned, deCaster’s and Pallemar’s bodies vanish. Ben, Tovel, Roba and Haunt then rush back into the control complex -- having been chased through the tunnels by the cherub statues, which have come to life. As the others try to build a barricade, Haunt collapses in pain as a formerly undetected tumour liquefies and is expelled through her pores. The Doctor theorizes that a process has been set in motion to drive impurities out of the soldiers’ flesh, and that this process also affected the interface between Shel’s organic and mechanical parts. Perhaps this is part of a plan to reanimate the Ten-Strong -- ten humans for ten Schirr? This theory seems confirmed when Joiks discovers that Frog’s flesh is changing, from that of a human into that of a Schirr...

Joikes insists upon killing Frog before she turns on them. Frog tries to flee, but Tovel stuns her and Haunt recovers long enough to order her team to work together. Tovel decides to check out the cavern of blue light which Polly saw earlier, and which the Doctor believes may house the complex’s drive. Polly stays behind to watch over Frog, Shade and Haunt. The porthole which Polly saw earlier acts as a marker, but once they clear the rockfall the humans are also mesmerized by the blue light. The Doctor resists the light’s hypnotic effect and snaps the others free of it before they walk into the propulsion units. There are already chunks of flesh whirling about inside the drives, powering the rituals which propel the complex through space -- both human and Schirr flesh, which suggests that the Morphieans are responsible for what’s happening here.

Polly catches Shade trying to erase files from Lindey’s palmscreen, and demands an explanation. He admits that he joined the ATE to prove that he wasn’t a pampered and priveleged Earthborn, but when his team ran into an ambush he fled in terror, running straight into a fragmentation grenade and leaving his men to die. Only the connections which he was running from saved him from a court-martial and execution, but he kept the scars to remind himself of his cowardice. He was allowed to re-enter service with a doctored record, but Lindey found out the truth and planned to blackmail him. Polly promises to keep his secret, but then discovers that she’s accidentally erased the files herself in any case. Frog then suffers a panic attack and tries to cut the Schirr flesh out of her body, but Haunt revives and stops her, refusing to let anyone else on her team die.

Creben finds two discarded websets near the propulsion units, but Joiks tries to destroy them. When pressed, he admits that he made a pass at Denni in exchange for supporting her bid to oust Haunt from command. The others suspect that he is in fact responsible for her “disappearance”, but are proven wrong when two stone angels fly in and attack, ripping Joiks apart and flinging his body into the drives. They retreat to the control chamber, shaken by the violence, but the Doctor considers it a hopeful sign; the Morphieans are acting like animals, trying to impose their will upon “weaker” species, and this means that they are flawed and can be defeated.

Frog discovers that she can now speak without her electronic vocoder, as the transformation is healing her childhood facial injuries. Roba panics when he discovers that he is also changing, and flees into the tunnels, terrified of what he’s becoming. Haunt lets him go, telling the others to let him cool down. They examine the websets which Creben found, and discover that one belongs to Lindey and the other to Shel, who was caught by the angels, torn apart and put into the drive. As an android, he interfaced with the neural net more effectively than any of the others, and they’re thus able to determine that he wasn’t responsible for setting up this trap at all. When he shot the control panel, he was trying to draw the Doctor’s attention to something near it. If Shel wasn’t responsible for setting up the trap, that means someone else must be -- and since Denni was the first to “disappear” and her webset still hasn’t been found, she would seem the logical suspect.

The Doctor studies the control panel which Shel shot, and realizes that some of the circuits are similar to those in the TARDIS. The “force field” is in fact a time stasis field, and a small crystal embedded in it enables the Schirr within to transmit commands to the outside. This must create time spillage, which explains why the Doctor can’t get back into the TARDIS and why deCaster and Pallemar seemed to vanish so quickly. But if the Schirr are responsible for what’s happening, why is there Schirr flesh in the propulsion units? Before the Doctor can form a theory, the stone angels attack again, but Shade successfully shoots one with a grenade launcher -- and it bursts apart into millions of fleas. The Morphieans have used the fleas as raw flesh with which to construct the angels, golems given form through the Morphiean rituals.

The complex’s life-support systems shut down, and the soldiers realize that they can’t risk splitting up to look for the controls as long as the angels are roaming the corridors. Haunt thus suggests using the websets to transmit rather than to record, so the soldiers can all keep an eye on one another regardless of how far apart they are. The Doctor rewires the websets, and he, Ben and Polly take Lindey’s, Shel’s, and Joiks’. This is not true telepathy, and the Doctor is able to block his thoughts -- although when Polly tries to read him she picks up the momentary, terrifying sensation of being a vital young mind trapped in an old, dying body. The soldiers set off in search of the life-support controls and the missing nav-crystals, but Frog must remain in the control chamber, as most of her flesh is now Schirr rather than human and she can barely move.

The Doctor teams up with Haunt, who sympathizes with his feelings of frustration when his old body is unable to keep up the pace. She once felt that way, when she and her partner Ashman were trapped on the colony world Toronto, injured by the blast from a grenade held by a dead Schirr. After days under the rubble, a communications unit told them rescue was on the way, and in that moment of triumph Ashman held out his hand to Haunt... but when she reached out to him he slapped her hand aside, demanding more painkillers. Haunt threw them away in a fit of rage, and Ashman later died of his injuries. Haunt pulls herself back to the present, and she and the Doctor continue on to find Roba, who is sitting paralysed in a tunnel -- with a webset on his head, where it was placed by one of the angels. Roba’s flesh is becoming more like that of the Schirr, and through him, the team sees the memory of one of the Ten-Strong and learn that they submitted willingly to the “death” of the stasis field -- all except for Pallemar, who feared deCaster’s ambition and betrayed them to Earth Central. Pallemar is the only one of them who was truly killed (presumably it’s his flesh in the drives). Haunt agrees to let Roba live, although it seems he’s becoming one of the Schirr. She and the Doctor proceed -- but Haunt sees movement up ahead, and when she investigates she is attacked by the angels. She is removed from the neural net as she orders her team to keep the net together.

Polly and Shade have teamed up, and Polly notices that the damage to Shade’s face is all but gone. She admits that she accidentally erased Lindey’s files; he has the opportunity to make a clean start... whether he wants it or not. When Haunt is removed from the network, presumably killed, this just convinces Shade that he’s a jinx. He and Polly find the porthole again, the stars twinkling like diamonds beyond, but the moment of tenderness between them is lost when Polly realizes that Shade is just like the other men she met at the London nightclubs, wounded and seeing her only as a girl who can make them feel better for a while. Ben is the only one who’s treated her as a person. She absently reaches out to the twinkling stars as if she could touch them... and pulls her hand back holding the missing nav-crystals, which had been hidden in plain sight all along. She and Shade contact the Doctor, who advises them to keep their discovery a secret, as they don’t know how far the Schirr influence extends through the net.

As Ben searches the tunnels with Tovel, Tovel seizes up, suddenly paralysed. Ben is unable to help him, and is forced to leave him where he lies and continue the search alone. Creben finds the life-support controls, and Ben joins him and helps him to repair them -- but they realize that it was too easy to do so. And why hasn’t Denni sent the stone angels to stop them? The Doctor realizes that the sabotage to the life-support system was a distraction to get them out of the control chamber, but he’s too late -- the angels have already deactivated the stasis field and released the Ten-Strong. The Schirr and the Morphieans are working together after all -- but even the Doctor is surprised when Haunt enters the control chamber and holds him at gunpoint, revealing that she is the traitor.

deCaster is surprised when the other humans enter the chamber, as the initial stages of the ritual are complete and they should all be paralysed. The Doctor claims to have stopped the paralysing pulse, but refuses to explain how unless the Schirr explain themselves. deCaster explains that casting the terror rituals has taken its toll on the Ten-Strong’s flesh, and that they planned to lure ten human soldiers here in order to purify their flesh and absorb it, thus restoring themselves to full strength. The initial tableau was meant to confuse the soldiers and set them investigating a non-existent mystery, while two Schirr were released to cast the preliminary stages of the ritual. Pallemar betrayed them to Pentagon Central, which was why Shel was sent in undercover -- and since deCaster killed Pallemar for his treachery, the Schirr only required nine bodies instead of ten, which was why Denni was killed. When the Doctor and his companions arrived, the presence of extra flesh threatened to destabilize the rituals, and the Morphieans angels thus culled more of the humans, picking them at random since they were unable to distinguish between them, and killing Joiks when Haunt stopped Frog from killing herself.

The Doctor realizes that these Morphieans do not represent the majority of their species -- like the Schirr, they are dissidents who refuse to tolerate the expansion of the Earth Empire, and who wish to experience the pleasures of corporeality once again. Most of the Morphieans don’t care about the activities of the human race -- but this will soon change, for when the Ten-Strong have cast their ritual and consumed the flesh of the humans, the rituals will enable the Morphiean dissidents to break open the mindforce of their race and take control. The Ten-Strong will then launch an attack on Earth, with the full backing of the Morphiean Quadrant, using their rituals to transform all humans into Schirr which the Morphieans can then possess.

As the complex nears the heart of the Morphiean Quadrant and the ritual nears completion, the rate of change begins to accelerate, and all of the soldiers find their flesh transforming into that of the Schirr. The angels bring the paralysed Tovel and Roba to the control chamber to await the completion of the ritual, while deCaster and Haunt drag the Doctor to the propulsion units to force him to release the paralysing pulse. Before he goes, the Doctor tells Polly that there’s still time to turn things around, and after some thought she realizes that he means this literally. As she’s recovered the nav-crystals, they can alter the course of the complex and pilot it back out of the Morphiean Quadrant. They must distract the Schirr first, for although the Schirr can’t afford to kill any of the remaining humans, the angels can still torture them. Ben and Shade use the force mattress capsules to create a distraction and flee, and as the angels pursue them, Polly, Tovel and Creben try to reinstall the nav-crystals.

The Doctor deduces that Haunt was “rescued” from the ruins of Toronto, not by her own kind, but by the Schirr, who planted the tumour in her body; even then, they were planning this, and they needed a traitor on the inside, and only by helping them to conduct this ritual could she get rid of the tumour. Blaming herself for Ashman’s death, she tried to make amends afterwards by killing as many Schirr as she could -- but she eventually understood that she too had become a terrorist, just killing innocent civilians and never touching the Morphieans. She wants the Morphieans to get bodies of their own so the humans can fight them properly; she was the one who suggested that the golems in the complex should take the form of angels, which would guide her and her people to their rest.

deCaster finally realizes why the Doctor is so weak -- he’s been holding back the paralyisng pulse with his own mind. The angels pummel the Doctor until his resistance breaks down and the paralysing pulse enters the network, freezing the humans in place. Haunt gives herself to deCaster, who consumes her body, regains his strength, and begins to cast the final ritual. Through the neural net, the Doctor senses his friends fading away -- and he also senses the presence of Shel the android, who interfaced with the net more effectively than the humans and has left a trace of himself inside. The Doctor reaches out to him and urges him to help them resist. The ritual thus goes awry, and the Schirr find themselves paralysed. Tovel nearly gives in to their influence and tries to kill Polly, but Creben fights him off -- and Roba, feeling himself change and unable to do anything about it, deliberately puts a force mattress capsule in his mouth and bites down on it. Seeing Roba’s gruesome death, Tovel pulls himself together and operates the navigational controls.

Roba’s death destabilises the ritual completely, and the energy feeds back into the bodies of the Schirr, killing them. The breaking of the ritual also enables the main Morphiean mindforce to strike back against the dissidents, and the angels disintegrate into fleas once more. Shade and Ben reach the propulsion units, and at the Doctor’s urging, Shade kicks the dying deCaster into the drive, feeding it with his flesh, giving the complex a boost of power and accelerating it out of the Morphiean Quadrant. The Doctor rigs up a transmitter and Shade programmes it to transmit a distress call; like Frog, he’s been given a second chance, and he intends to use it properly. As there is no more time spillage, the Doctor and his companions enter the TARDIS and depart, hoping that the survivors will learn from their experiences and treat the remaining Schirr with compassion and understanding, preventing any further such atrocities.