The Doctor lands the TARDIS in Martha’s bedroom on Earth, 12 hours after she left with him. He prepares to leave when Francine, Martha’s mother, calls and says that Tish is on the TV. They tune in to a press conference where Professor Richard Lazarus, Tish’s employer plans to demonstrate his new device that evening, a device that will “change what it means to be human.” The Doctor and Martha exchange awkward goodbyes and he leaves… only to rematerialize several seconds later and suggest they investigate Lazarus.
Lady Thaw is meeting with the elderly Professor Lazarus and he refers to her employer, Mr. Saxon. Tish, Lazarus’ PR head of staff, arrives and he hits on her without much luck. The Doctor and Martha arrive and get in on Martha’ invitation, and find Tish. Francine, and Leo Jones arrive. Francine is immediately suspicious of Martha’s new “friend.” Professor Lazarus arrives and shows off his device, a large sonic-based regeneration cabinet, and then steps into it. The device begins to spin and then go out of control. The Doctor leaps to the controls and manages to shut them down before they overload and destroy the building. Lazarus emerges from the chamber… 50 years younger.
The new Lazarus starts eating everything in sight while the Doctor points out to him that he couldn’t have predicted all the variables. Lazarus dismisses his concerns and boasts that they will soon be selling the process commercially. He then kisses Martha’s hand and leaves with the elderly Lady Thaw. The Doctor decides to use one of the laboratories in the building to run some tests and Martha offers the DNA in Lazarus’ saliva on her hand.
Lazarus and Lady Thaw chat about their experiences in the Blitz while admiring the nearby cathedral. They start to kiss but then Lazarus draws back, disgusted by the elderly Thaw. Meanwhile, the Doctor has completed his analysis and determines that Lazarus’ DNA is mutating even as they watch. He goes to find the scientist, while Lazarus goes through a series of convulsions then mutates into a hideous scorpion like creature that attacks Thaw.
The Doctor and Martha arrive and find that Thaw has been completely drained of her life force and turned into a withered husk. Downstairs, a now-normal Lazarus finds Tish and invites her upstairs. They just miss Martha and the Doctor and go up to the roof to admire the roof. The Doctor quickly finds out from Francine where Tish is and he and Martha go up to the roof, while a mysterious man talks to Francine about how dangerous the Doctor is.
The Doctor uses his sonic screwdriver to track Lazarus mutating DNA and goes to the roof. He gets Tish away from Lazarus, who mutates again and attacks them. They manage to seal off the rooftop door but Lazarus’ efforts to get through set off the security system and cause a lockdown. They get downstairs and try to get the guests out, and the guests panic as Lazarus arrives. Martha gets the doors open using the sonic screwdriver while the Doctor lures Lazarus off into the basement area. He then slips into a laboratory and opens all the gas valves, setting off an explosion which only slows the monster down. The Doctor meets up with Martha again and they run into the party room to take refuge in the regeneration device. Lazarus is hesitant to destroy it to get at them but activates the device. Working desperately, the Doctor reverses its polarity, throwing the regeneration wave outward and knocking Lazarus unconscious while causing him to turn back to human form.
The ambulance drivers take Lazarus away while an angry Francine slaps the Doctor. There’s a nearby crash and the Doctor goes to investigate, and finds the EMTs drained of their life force. He goes to the nearby cathedral and Martha and Tish insist on going with him. As they leave, the mysterious man from the party warns Francine of exactly what the Doctor is.
In the cathedral, the three of them find Lazarus seeking sanctuary in the same cathedral where he sought safety during the Blitz. The professor is unrepentant, boasting of how the desire for life is humanity’s greatest gift. The Doctor warns that the only thing immortality assures is loneliness, but Lazarus ignores him and mutates again. Martha and Tish lure him up into the belltower while the Doctor plugs his sonic screwdriver into the cathedral’s pipe organ and pulls out all the stops. Martha dangles precariously from the catwalk as the Doctor generates a sound wave that disrupts the sonic process that created the Lazarus mutant. It falls to its death and reverts first to young Lazarus, and then the aged form.
Later the Doctor and Martha return to Martha’s bedroom. Martha eventually prods the Doctor into taking her on one more trip, as a full-fledged crewmember. They leave just as Francine calls again, warning that the Doctor is a menace to them all according to the information given to her by Mr. Saxon.