Fenric was trapped by the Doctor for 1700 years but is now free. He blasts through the window and vanishes. When the guards come, Millington orders the Doctor, Ace and Sorin shot.
Sorin's men attack the execution squad, freeing the Doctor, Ace and Sorin. As the firefight rages, the Doctor tells Ace he used a game of chess to trap Fenric.
Fenric meets with Phyllis and Jean. He tells them to summon the Ancient One.
Sorin leads the Doctor and Ace to Millington's office to get his chess set.
Phyllis and Jean summon a massive haemovore out of the water.
Ace tries to take Millington's chess set, but it is trapped with poison and explosives. They dash out just before the room explodes.
The Ancient One meets with Fenric. His world is dead but Fenric taunts him that it won't become his world for a long time.
Ace tells the Doctor that Dudman has a chess set.
The WRNS are attacked by Fenric while the British soldiers use gas grenades to finish off the Russians. Vershinin and Sorin, the only survivors, decide to destroy the ULTIMA machine.
Captain Bates overhears Fenric telling Millington there is enough poison in the bunker to contaminate the world forever. He races out and tells the Doctor, then offers to join forces with Sorin against Fenric. Sorin gives his soviet star to Ace.
The soldiers find the WRNS all turned into vampires and are killed.
The Doctor and Ace find Dudman, who indeed has a chess set. Ace stays to protect her and baby Audrey while the Doctor deals with Fenric.
Fenric has his haemovores kill Nurse Crane. Ace and Kathleen barricade themselves into her quarters. Vershinin tries to destroy ULTIMA but is caught and shot by Millington. The Doctor hurriedly sets up a chess set in the laboratory.
Haemovores attack Ace and Kathleen. Ace leads Kathleen and the baby out of a window. She gets Kathleen into a truck and tells her to go to her aunt in London. She kisses the baby goodbye and Kathleen races off. But Phyllis and Jean have found Ace.
Fenric tell the Ancient One to release the poison into the water. Having no use for the other haemovores, he orders them killed. The Ancient One destroys them all, including Jean and Phyllis.
Fenric finds the Doctor and is offered a chess problem: one move to win from a set position of kings and pawns.
Bates comes to Vershinin's aide, but is confronted by Millington. Bates is unable to shoot his commander, but Vershinin is.
As Fenric tries the problem, the Doctor meets the Ancient One. The haemovore comes from an Earth reduced to a chemical slime thousands of years in the future. Fenric took him back in time to destroy the earth and create his own future.
Ace finds Fenric delirious, unable to solve the problem. He demands the solution and she races out. She finds Vershinin and Bates, who say they are pawns who have joined forces. Ace realizes this is the solution to the chess problem.
Sorin finds Fenric and points his gun at him. Fenric realizes that Sorin was selected for the mission because his grandmother was english, descended from the Viking settlers. Sorin is one of the wolves of Fenric.
Ace arrives later to find Judson dead. She tells Sorin the solution only to realize too late that he has been taken over by Fenric.
As the Doctor returns, Fenric boasts that everyone has been a pawn in his game. All the descendants of the Vikings were his pawns: Sorin, Millington, Judson, The Ancient One and… Ace. He tells Ace that Kathleen's baby is, in fact, Ace's own mother.
He tells the Ancient One, who has returned, to kill the Doctor and Ace. But Ace has faith in the doctor and her psychic barrier keeps the Ancient One back. Fenric then holds a vial of poison over Ace and says he will kill her unless the Doctor kneels before him.
The Doctor tells him to go ahead and kill her. He's long known that Ace was a pawn of Fenric's and only took her on because of it. His insults break Ace's faith and she bursts into tears. Freed of her psychic barrier, the Ancient One picks up a container of poison and instead advances on Fenric. He forces him into the isolation booth and releases the poison, killing them both.
The Doctor drags Ace out before the bunker explodes. He assures her that he only insulted her to break her faith. He knew the Ancient One would destroy Fenric, given the chance.
The Doctor and Ace return to Maiden's Point. She is torn between her love of Katheleen's baby and her hatred of her adult mother. She dives into the water and emerges feeling liberated. They return to the TARDIS.