On a cold, wet day in the docklands of London, a group of terrified men burst out of a warehouse into the streets -- only to be gunned down in cold blood by two police constables, who also shoot an elderly witness who happened to be passing by. A man in the uniform of a detective inspector, Lytton, operates a hand-held device which transports both himself and the bodies to a distant spaceship; there, he prepares to lead an attack on an unsuspecting space station. Back on Earth, two survivors of the massacre, Galloway and Stien, return to the warehouse, where Galloway hopes to use the time corridor to warn his own people; instead, he is gunned down by a trooper hiding in the shadows. Stien hides outside, where he sees a squad of soldiers arrive at the warehouse.

The TARDIS has been caught in a time corridor, and the Doctor's attempts to break free result in severe strain on the ship. He manages to escape eventually, and the TARDIS materializes in the docklands, near the exit to the time corridor. The Doctor sets off to investigate, followed by Tegan and Turlough, and they run into Stien near the warehouse. Stien, seemingly terrified and on the point of collapse, warns them away from the warehouse and the squad of soldiers, but the Doctor's curiosity has been piqued and he ventures inside.

Centuries in the future, young Lieutenant Mercer arrives to take up his new posting on a prison station in deep space, where he is appalled by the poor state of repairs and the lacklustre performance of the crew. The captain isn't fit to hold his position, but nobody is willing to stand up to him. The station medic, Styles, warns Mercer not to stick his neck out, but he makes a formal complaint nevertheless and thus finds himself assigned as Officer of the Watch on graveyard shift. His fellow officers have grown bored with the endless, dull routine... and are thus caught entirely off guard when Lytton's battle cruiser emerges from warp space and opens fire. The first salvo instantly takes out the station's defenses and kills half the crew, including the captain.

The battle cruiser heads straight for the one airlock which has yet to be repaired, and Mercer, suddenly the senior officer aboard the station, tries to organize a defense. The surviving crewmen manage to mine the corridor before the airlock is blasted open -- and find themselves facing a squad of Daleks. The crewmen manage to drive off the first wave of attackers, just as Lytton had expected, and the Black Dalek in charge of the offensive reluctantly authorises Lytton to proceed with his own plan. Lytton sends his troopers to release gas canisters into the station; the corrosive gas burns the skin off the defenders' flesh before they can react, and Mercer, Styles, and the only two other survivors are forced to retreat as Daleks and mercenaries pour into the station.

Mercer gets to the bridge and tells the crew to abandon ship, before he, Styles and the other two troopers flee into the depths of the station. His fellow officer Osborn attempts to carry out her final duty and execute the prisoner, but due to systems failure she must set off with another officer to complete the task in person. Moments later, the Daleks reach the bridge and slaughter everyone inside. Osborn and her fellow officer manage to reach the cryogenics chamber, but some of the Daleks' corrosive gas has drifted this far, and the officer finds the flesh melting away from his hands and face. Osborn, terrified, guns him down as he begs her for help, but the noise attracts the Daleks' mercenaries, who enter the chamber and kill Osborn before she can key in the final commands to kill the prisoner. Lytton, satisfied that the station is secure, releases the prisoner... the cryogenically suspended Davros.

The Daleks, having detected the Doctor's presence in the warehouse, send one of their own to capture him. Turlough, meanwhile, stumbles across the entrance to the time corridor and is transported to the Dalek ship, where he is forced to hide from the approaching Dalek. The Doctor notices that he's gone, but before he can do anything he, Stien and Tegan are captured by the bomb disposal squad led by Colonel Archer. The Doctor tells Archer what he's really doing here and is surprised when Archer doesn't dismiss his story out of hand -- and when he meets Archer's scientific advisor, Professor Laird, he realizes Archer must have found alien artefacts in the warehouse. Archer tries to contact his HQ for further orders, but can't get through due to interference from the time corridor. And then, much to the Doctor's horror, the time corridor opens and a Dalek emerges... [1]

The soldiers open fire, but one is gunned down before the Doctor can tell the others to shoot out the Dalek's eyepiece. The soldiers then push the blinded Dalek out of the loading bay, and it smashes to pieces when it hits the street three metres below. Tegan has been struck in the head and Laird takes her to rest. Archer shows the Doctor a number of canisters found by construction workers preparing to demolish the warehouse; they appeared to be unexploded bombs left over from the Blitz, but have turned out to be alien artefacts. Archer, realizing that he's out of his depth, gives his gun belt to the Doctor with orders to help his men fight any further Dalek intrusion, and sets off into the streets to find a working telephone with which to contact his HQ and request reinforcements and an ambulance for Tegan. But once he is out of sight of the warehouse he is captured at gunpoint by the two "police constables"...

Turlough, cut off from the time corridor by a Dalek patrol, makes his way through a room full of corpses and a cloning chamber beyond. The Daleks are aware of his presence but have decided to let him roam freely, as he will serve as bait to lure the Doctor to their ship. Turlough manages to find his way out of the Dalek ship onto the prison station, where he runs into Mercer, Styles and the two other survivors -- who are searching for the station's self-destruct chamber. Mercer doesn't believe Turlough's claim to have come through a time corridor, but Styles is more open-minded; nevertheless, they can't let him wander about freely now that he's seen them, and they force him to accompany them.

Lytton summons an engineer, Kiston, to help revive Davros, and once fully awake Davros is appalled to learn that the Daleks have lost their war with the Movellans and now require human troopers to augment their forces. The Movellans discovered a virus which attacks Dalek tissue, and now the Dalek Empire has been reduced to a few scattered colonies seeking a cure. This is why they have returned for their creator; but this time Davros does not intend to let them abuse his genius, and he insists upon remaining on the station to work. Lytton, taken aback, reports to the Black Dalek, who orders him to play along with Davros for the time being; but while Lytton is out of the room, Davros jabs Kiston with a device which enslaves him to Davros' will. Daleks arrive to escort Davros to Styles' laboratory, while Davros obsesses over revenge for his 90 years of imprisonment; not only on the human race, but on the Doctor.

The soldiers bring the scattered bits of the destroyed Dalek back into the warehouse -- but the soldier left to guard the debris soon finds out that the embryo inside is not quite dead... The Doctor is trying to convince Sergeant Calder to let him return to the TARDIS and rescue Turlough when they hear the screaming, and must hunt down the Dalek embryo and kill it. As Calder and Laird take the wounded soldier to recover and wait for the ambulance to arrive, the Doctor takes the opportunity to return to the TARDIS and forces Stien to accompany him and help locate the Dalek ship. Soon afterwards, the wounded soldier -- possibly affected by the presence of alien toxins in his bloodstream -- leaps from his chair and runs back upstairs. Calder follows, only to be surrounded by four Daleks which have emerged from the time corridor. Tegan and Laird, hearing the gunfire, try to flee, but are prevented from leaving the warehouse by Archer, Calder -- and the two other soldiers, including the one who was killed earlier. And Archer is now armed, although he'd given his gunbelt to the Doctor before leaving. When Laird asks how long the ambulance will be and suggests taking Tegan to hospital herself, Archer coldly announces that the warehouse is now under martial law -- and if they try to leave, they will be shot.

The Daleks enter the terminal and surround the Doctor, their lifelong enemy finally at their mercy... until Lytton arrives to remind them that the Dalek Supreme wants the Doctor alive. The Daleks usher the Doctor off to the duplication room, while Lytton and Stien muse that the Daleks will kill anyone on impulse, even people they need alive. The Doctor, having seen the discarded bodies of the soldiers from the warehouse, tricks Stien into admitting that Davros is present; however, he still doesn't know why the Daleks need their creator.

Laird's attempt to slip Archer a drugged drink fails, and then Archer finds the gunbelt which the Doctor left behind. Suspicious, he takes it to show the others, and Tegan and Laird take the opportunity to make a dummy in the cot, using pillows and one of the alien canisters, so Tegan can escape and fetch help. Before she can go, Archer returns to announce that the charade is over; they are to be sent to the Dalek ship as soon as it's convenient. As soon as he's gone, Tegan flees; Laird remains behind to maintain the illusion, as the dummy isn't terribly convincing.

Davros establishes himself in Styles' laboratory, and requests the help of a chemist and samples of the Movellan virus. The Daleks alter the location of the time corridor to fetch a canister from the warehouse, creating a high-pitched sound which temporarily incapacitates Laird. When Archer and Calder come to investigate they realize immediately that Tegan has escaped. Meanwhile, Davros converts both the chemist and the two soldiers carrying the virus canister to his cause, as well as the two Daleks who have arrived to provide him with tissue samples for his experiments...

Turlough is appalled to discover that Mercer and his officers intend to blow up the station with themselves on board, and suggests that they prime the self-destruct mechanism and escape through the time corridor. Mercer accompanies him to determine whether his plan is feasible, but while they're gone the Daleks detect the intruders in the self-destruct chamber and send Lytton and his troopers to deal with them. Styles and the others barricade themselves within the chamber, and with nothing to lose, Styles begins to tinker with the self-destruct mechanism, trying to activate it.

The Doctor learns that all of the Daleks' mercenaries, including Stien, are duplicates -- and that after duplicating him and his companions, the Daleks intend to send them to Gallifrey to assassinate the High Council. The time corridor was just one of many traps set for him. The Daleks leave to carry out other duties, leaving two troopers on guard outside the duplication chamber while Stien straps the Doctor into the mind analysis machine. But the Doctor realizes that Stien is uncomfortable with what he's doing -- and that old memories from before his duplication are still present behind the Dalek conditioning. The Doctor desperately tries to access them and restore Stien's sense of self, but the Dalek conditioning is too strong -- although he makes some progress, Stien activates the mind analysis machine, driving the Doctor's mind back through his memories and recording them on tape.

Tegan flees back to the docks, only to find the streets deserted and the TARDIS gone. When she sees two policemen nearby she approaches them for help -- and then flees when one of them pulls out a gun with a silencer attached. Trapped by the pier, she sees a man scanning the riverbed with a metal detector. He doesn't hear her calling for help -- but much to her horror, when the two policemen arrive, they shoot him anyway. They then march Tegan back to the warehouse, where Laird panics and makes a break for it, only to be gunned down by Archer. The other soldiers force Tegan into the time corridor.

Mercer and Turlough find the airlock too heavily guarded to get through, and Mercer forces Turlough to return to the self-destruct chamber -- only to find Lytton preparing to break in. Turlough points out that the Daleks must be trying to prevent the destruction of the station because Davros is still on board. He and Mercer search the station and eventually locate him in Styles' laboratory -- which is also too heavily guarded to attack. They return to the self-destruct chamber but arrive too late; Styles has primed the self-destruct mechanism, but Lytton and his troopers break in and gun down her and the other two prison officers before they can detonate it. Turlough tells the devastated Mercer that he's done all he can; their only choice now is to reach the time corridor and escape. Meanwhile, the Daleks blame Lytton for failing to predict the self-destruct attempt, although he points out that the original plan was to get Davros off the station as soon as possible.

Davros and his servants, meanwhile, are filling small capsules with samples of the Movellan virus. Soon Davros will exact vengeance upon the Doctor -- and then he will dispose of the Daleks himself, to create an entirely new race, even deadlier than the old, with Davros as their rightful leader. [3]

Stien snaps and switches off the mind analysis machine, saving the Doctor's life. He and the Doctor overpower the guards and strap them down in the machine, but Stien remains aware that the Dalek conditioning could cloud his mind at any moment. Meanwhile, Tegan arrives on the Dalek ship just as Mercer and Turlough arrive at the time corridor's terminal; but they see the TARDIS nearby, indicating that the Doctor must be on board somewhere. Forced to hide from a Dalek patrol, they find the duplication room and are reunited with the Doctor. The Doctor destroys the tape of his recorded memories and takes them all back to the TARDIS, where Stien explains about the Movellan virus. The canisters were stored on Earth to prevent their accidental release aboard the Dalek ship, placed in an era where they would lure people who could be duplicated and left to guard them without raising suspicion. To Tegan's horror, the Doctor decides that he has no choice but to kill Davros. He programmes the TARDIS to take Tegan and Turlough back to Earth if he doesn't return and sets off with Mercer and Stien. Moments later the TARDIS dematerializes of its own accord, and Tegan and Turlough realize that the Doctor wasn't expecting to return.

Davros sends his Daleks and two troopers to seize the TARDIS, and gives viral capsules to his chemist and the other trooper to release aboard the Dalek ship. The Doctor arrives, apparently guarded by two troopers -- who are in fact Mercer and Stien in disguise. As the Doctor prepares to shoot Davros, Davros claims that he was going to re-engineer the Daleks to give them compassion and pity; but the Doctor knows he only intends to make them better warriors. Davros insists that death, struggle and war are universal constants, a viewpoint the Doctor rejects -- and even with a gun pressed up against Davros' head, the Doctor is unable to pull the trigger.

The Daleks detect the Doctor's escape and also discover that Davros has been bringing troopers and Daleks under his direct control. Davros cannot be trusted, and therefore must be exterminated. Davros' servants have gone to Earth in pursuit of the TARDIS, and the Daleks order Lytton to follow and kill them -- but they also believe that Lytton is growing too unreliable and arrogant, and send a squad of Daleks after Lytton's troopers, to exterminate any survivors.

Two troopers arrive at Styles' laboratory to kill Davros, but, unaware of their mission, the Doctor sends Mercer and Stien to intercept them. Mercer shoots the two troopers, but as Stien tries to help him hide the bodies his Dalek conditioning begins to reassert itself. When two more troopers arrive Stien momentarily identifies them as allies -- and they are thus able to open fire first. The Doctor rushes out to help too late; the troopers are dead, but so is Mercer, and Stien is badly injured. He staggers off into the depths of the station, ordering the Doctor not to follow him; and when the Doctor turns back to the laboratory, he finds the door has been closed and sealed behind him. Having lost the opportunity to kill Davros, his only choice is to use the Dalek time corridor to return to Earth.

The TARDIS materializes in the warehouse, where Tegan and Turlough steal a canister of the Movellan virus. The soldiers are occupied fighting Davros' Daleks upstairs; they are all killed, but then Lytton's squad arrives, followed by the Daleks loyal to the Dalek Supreme. By the time the Doctor arrives total war has broken out between the three factions, but he manages to clear a path to the door using explosives from the Dalek ship. Reunited with Tegan and Turlough in the TARDIS, he opens the viral canister -- which only affects Daleks, not humans -- and releases it in the warehouse. Most of the human troopers are already dead except for Lytton, who kills the one surviving trooper so as not to be burdened with him and sets off into the streets of London with his "policemen". The virus then makes short work of the remaining Daleks.

Davros' chemist and trooper are exterminated before they can release the virus onto the Dalek ship, but Davros has already released it within his laboratory. When two Daleks arrive to exterminate him they fall victim to the virus within seconds. Davros triumphantly heads for an escape pod to begin his work elsewhere -- only to find too late, as his chair judders out of control, that he too is susceptible to the virus... Meanwhile, Stien drags himself to the self-destruct chamber, kills the two guards on duty and re-primes the mechanism. The Black Dalek detects his presence and sends Daleks to kill him, but they arrive too late -- although they manage to gun down Stien, his last act is to throw himself onto the self-destruct switch...

The Black Dalek contacts the Doctor in the TARDIS, to announce that though the Doctor has won this battle, the Daleks' duplicates are still placed in strategic positions throughout Earth's governments. The collapse of Earth society is inevitable -- but then the Dalek is cut off in mid-rant as the prison explodes and takes the Dalek ship with it. The Doctor feels certain that the duplicates' conditioning will eventually wear off as it did with Stien. But as he prepares to leave, Tegan announces that she won't be coming with him; the death and horror have finally become too much for her. She bids the Doctor and Turlough a tearful goodbye and bolts for the door, stepping over bodies as she goes. The Doctor, upset, admits to Turlough that he left Gallifrey for similar reasons.