In present-day London, an elderly man named Professor Richard Lazarus announces on television that he will demonstrate a device that will change what it means to be human. Intrigued by this statement, the Tenth Doctor joins Martha to go to the launch party at Lazarus Laboratories, where they meet up with Martha’s sister Tish, who works there. Lazarus steps inside a capsule, and emerges as a much younger man. Using a sample of Lazarus’ DNA, the Doctor and Martha find that Lazarus had successfully managed to instruct his genes to rejuvenate with sound waves, but also activated something in his DNA, which is trying to change him into something else.
Meanwhile, Lazarus returns to his office with his partner, the elderly Lady Thaw. She insists that she be the next to use the machine so they can be young together, but he refuses. She threatens to have Mr. Saxon pull their funding, but Lazarus transforms into a monster and kills Lady Thaw. The Doctor and Martha discover Lady Thaw’s body and deduce that Lazarus must drain life energy to keep his DNA stable. Lazarus attacks the Doctor and Martha, in his alternate form as a giant skeletal scorpion-like being, and they hide in his machine. The Doctor explains that Lazarus’ transformation is the result of an evolutionary throwback locked away in dormant genes that the machine unlocked. Lazarus activates his machine, but the Doctor sets the capsule to reflect energy rather than receive it, and Lazarus is blasted away.
Lazarus’ body is taken away in an ambulance. Martha’s mother Francine becomes doubtful about Martha’s connection with the Doctor after being informed by Harold Saxon that the Doctor is dangerous. The Doctor hears the ambulance crash and finds that the drivers have been drained of life. The Doctor, Martha and Tish chase Lazarus to the nearby Southwark Cathedral, where Lazarus sought sanctuary during the Blitz. Martha and Tish lure Lazarus to the top of the Cathedral’s bell tower, and the Doctor manipulates the church’s pipe organ to produce the maximum volume it can. The vibrations caused by the organ interfere with Lazarus’ manipulated DNA and he falls to his death.
The Doctor invites Martha to come along for one more trip. She refuses, saying she doesn’t want to travel with him as just a passenger. The Doctor agrees that she is more than that to him, and they leave together in the TARDIS.