The Haddron Republic is torn apart when the popular Consul General, Hans Kesar, declares himself General for life -- an Emperor in all but name. His two closest friends, Consul Milton Trayx and millionaire Gerhart Rutger, aren't quite sure what to make of this, until Senator Mathesohn -- an outspoken opponent of Kesar -- informs Trayx that Kesar has lost the entire Fifth Legion. Trayx thus speaks out against Kesar and calls for his impeachment. The result is a civil war, and although Kesar is eventually defeated, millions of lives have been lost. It is felt that execution would make Kesar a martyr, and he is instead sentenced to life imprisonment; but as he leaves the trial chambers a bomb explodes in the corridor outside...

Years later, the TARDIS materializes on the asteroid fortress Santespri, where Kesar, his former general Cruger, and a retinue of their most loyal soldiers are confined with a small force of loyal Republican soldiers to guard them. Due to the injuries inflicted by the bomb, Kesar is forced to wear a mask which completely covers his face, and his voice is filtered through an electronic synthesizer. Kesar and his troops are generally given the run of the prison, although they are confined to their quarters by night; in fact, Kesar's troops are often allowed to join the patrols in order to feel useful. Two soldiers on opposite sides during the war, Darkling and Hedan, have now fallen in love, although Darkling suspects he may be the soldier who killed Hedan's brother in battle...

Kesar and Cruger often play at chess, and tonight they play on remote holographic boards while confined to their rooms. Kesar defeats Cruger, who often sacrifices the long-term strategy for the sake of a short-term win; tonight, Cruger plays almost with the predictability of a machine. Elsewhere, a hooded figure attends a secret meeting with Remas, a guard who wants to buy his way out of his commission, but rather than pay for the device which Remas is supplying, the figure murders him and takes the device from his body. The TARDIS materializes shortly afterwards, and although the Doctor and his companions are initially suspected of the murder, surveillance tapes prove their innocence (although the tapes were unable to capture the face of the actual killer).

Trayx arrives on Santespri with his wife Helena and his android aide-de- camp Prion. Some of Kesar's former allies are still trying to raise public support on Haddron, and Trayx suspects that Mathesohn, no longer afraid of making him a martyr, has hired an assassin to kill Kesar. Trayx shows the Doctor about Santespri, including the Stardial Chamber, an observatory where the Doctor notices a star moving out of alignment. He casually mentions that he detected a subspace signal being sent from Santespri when he arrived; and moments later, the fortress' communications system is jammed, and as the "star" moves closer it is soon identified as a battle cruiser. The prison staff try to launch a distress beacon, but it is shot down -- and when the cruiser returns to its approach vector without pausing to confirm the kill, this indicates that it is crewed by VETACs -- a legion of robots which cannot possibly be defeated by the skeleton staff of Santespri. Perhaps Senator Mathesohn has sent an entire legion to kill Kesar so the blame will fall on Trayx. Or perhaps they're here to rescue him; but if this is the case, the Doctor wonders why Kesar doesn't seem pleased by the prospect...

As the prison staff prepares for battle, Victoria finds a man tampering with the suits of armour in the banqueting hall. He attacks her and leaves her for dead, but she survives and identifies him as Sponslor, a member of Kesar's retinue. The Doctor impersonates a torturer and tries to frighten Sponslor into explaining his actions, and then leaves Sponslor to stew while he plays a game of virtual chess with Cruger -- who's resting in his quarters, waiting for the attack. But while they are occupied, someone breaks into Sponslor's cell and murders him. Meanwhile, Helena Trayx contacts Kesar and begs him to keep their past affair a secret; she has come to realize it was a mistake and doesn't want to do anything to hurt her husband.

The prison staff's attempt to infect the VETAC command net with a viral toxin fails, and the VETACs land and seize control of Santespri. Kesar and his retinue are released to join in the defense of the station, but as they consolidate in the banqueting hall the suits of armour come to life -- they are actually dormant VETACs which Sponslor switched over to the invaders' command frequency. Only a handful of survivors escape the resulting slaughter, and shelter themselves in the prison cells. The route back to the TARDIS has been blocked, but Helena and Victoria can still escape to Trayx's shuttle to warn Haddron what is happening here.

Cruger offers to take Hedan and Darkling to the armoury to fetch a virus which could destroy the invaders, but after he's gone, the Doctor works out that Cruger is in fact the killer. He'd programmed his virtual chess board to play by itself, making it appear that he was still in his room while he murdered Remas and Sponslor; Remas must have supplied him with the materials which enabled him to summon the invaders to Santespri. Cruger sets off to contact the invaders, ordering Hedan to kill Darkling for the good of the Empire; uncertain of his motives, she only stuns Darkling, who soon recovers and follows -- although he's suffering from a mild concussion. He manages to contact Trayx and reports that the invaders are in fact the Fifth Legion -- the legion whose disappearance provoked the civil war. Matters then become more urgent as the VETACs capture the escaping Helena and Victoria, and threaten to kill them if Trayx doesn't surrender Kesar within the hour.

Cruger gives Prion's command frequency to the VETACs, but the Doctor is able to stop Prion from murdering Trayx and adjust his command frequency. The Doctor decides to try the same trick in reverse, and Jamie helps to capture a VETAC and remove its command circuit. The Doctor takes Prion to the Stardial Chamber to access the stored solar power there and jam the VETAC command net. Meanwhile, Hedan questions Cruger's strategy, and realizes that he deliberately sent the Fifth Legion into reserve to weaken Kesar's influence so the Imperial forces would move to make Cruger their Emperor instead. She reveals the truth to the VETACs, who still owe their loyalty to Kesar first -- but as they turn on Cruger and attempt to kill him for his betrayal, the Doctor and Prion send the disorienting pulse, temporarily disabling them. Cruger flees, murdering Hedan when she tries to stop him -- and the still-concussed Darkling is unable to reach them in time to save her.

Jamie and Trayx rescue Victoria and Helena while the VETACs are still out of action, and they retreat back to the prison cells. There, the Doctor confronts Cruger, having realized that he set the bomb which injured Kesar in the hope of making him a martyr. The VETACs recover and break through to the cells, where Trayx is forced to surrender -- and the man behind the mask is forced to reveal that he is really Gerhart Rutger. The real Kesar did indeed die in the bombing, and Rutger took his place to prevent Kesar from becoming a martyr. Kesar's death puts Cruger in charge of the VETAC forces, but when the gloating Cruger reveals that his agents are behind the growing Imperial movement on Haddron this triggers a hidden subroutine in Prion's programming. Prion has been subverted by Mathesohn -- he is the secret assassin whose existence Trayx had suspected, and as Kesar is no longer a viable target Prion must fulfil his programming by killing Cruger. Darkling gives his life to hold back Cruger while Prion shoots him, and with Cruger's death the VETAC chain of command reverts to Trayx.

Trayx and Rutger decide to report that Kesar died fighting the invaders, and to use this story to bring down Mathesohn. The Doctor gives them suggestions on how to turn their dying republic into a commonwealth to ensure its peaceful survival. As the Doctor and his companions depart, Rutger tries to tell Trayx about his wife's past indiscretion, only to learn that Trayx already knows and has forgiven her. But this raises a question; did Trayx really speak out against Kesar because of the loss of the Fifth Legion, or did he have more personal motives?