Doctor: David Tennant (Tenth Doctor)
Companion: Freema Agyeman (Martha Jones)
Gugu Mbatha-Raw – Tish Jones
Reggie Yates – Leo Jones
Adjoa Andoh – Francine Jones
Mark Gatiss – Professor Lazarus
Thelma Barlow – Lady Thaw
Lucy O'Connell – Party Guest
Bertie Carvel – Mysterious Man
Directed by Richard Clark
Written by Stephen Greenhorn
Script editor Simon Winstone
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 3.6
Series Series 3
Length 45 minutes
Originally broadcast 5 May 2007
The Doctor returns Martha to her flat, twelve hours after she first stepped in the TARDIS. They listen to a message from Martha's mother, Francine, informing her that her sister Tish is on TV. They watch a news report that shows an elderly man named Professor Richard Lazarus who claims that he will change what it means to be human. The Doctor leaves briefly in the TARDIS, but immediately returns, intrigued by Lazarus's statement.
The Doctor and Martha go to the launch party at Lazarus Labs and meet up with Tish, who works there. Francine and Martha's brother, Leo, also join them, and Francine is immediately suspicious of the Doctor's interest in Martha. Their conversation is interrupted as Lazarus announces he will perform a miracle and steps into a capsule in the center of the reception room. The machine starts and surrounds the capsule with shining white light, and the Doctor quickly steps in when the system begins overloading. He deactivates the machine, and Lazarus emerges from the capsule as a much younger man. The machine appears to be able to manipulate the subject's DNA to make them younger, but the Doctor is concerned about unknown side effects. Martha points out that they obtained a DNA sample when Lazarus kissed her hand earlier, and they race to Lazarus's lab to examine the DNA. They find that it is fluctuating and unstable; Lazarus had successfully managed to instruct his genes to rejuvenate, but has activated an unseen element hidden within his DNA which is trying to change him into something.
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 a "Mr. Saxon" pull their funding, but Lazarus transforms into a huge scorpion-like being which kills Lady Thaw. He returns to human form and to the reception, while the Doctor and Martha discover Lady Thaw's body and deduce that Lazarus must drain life energy to keep his DNA stable. Lazarus goes up to the roof with Tish and they talk for a few minutes until the Doctor and Martha run in and warn Tish. Lazarus and the Doctor argue over his rejuvenation; Lazarus sees it as a means of improving humanity by purging it of mortality, while the Doctor believes that he has broken Nature's one unbreakable law and will soon face consequences. While Tish and Martha argue, Lazarus transforms again and attacks them, but they manage to escape to the lift. The building security system locks down the entire building. The Doctor, Martha and Tish take the stairs, warn the guests of the danger and are quickly followed by the monstrous Lazarus. The Doctor gives Martha the sonic screwdriver so she can unlock the security doors and Lazarus kills a guest while the others flee in panic. The Doctor attempts to reason with Lazarus while being chased, but ends up trapped in the machine with Martha. The Doctor explains that Lazarus's transformation is the result of an evolutionary throwback locked away in dormant genes that are now becoming dominant; evolution had rejected the mutation for humanity millions of years prior but, rather in the manner of Pandora's box, Lazarus had unwittingly unlocked it. Lazarus manages to activate his machine but the Doctor reverses the polarity, causing the capsule to reflect energy outwards in a violent shockwave, blasting Lazarus away. They emerge to find Lazarus in his human form, naked and apparently dead.
As police and medical workers arrive to take care of the wounded, Lazarus's body is taken in an ambulance. Francine becomes doubtful with Martha's connection with the Doctor after being informed by Harold Saxon that the Doctor is dangerous but is unable to convince her to part ways with him. 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. The Doctor tries again to reason with Lazarus, who reveals that his experiments were a result of experiencing the horrors of the London Blitz as a child and vowing not to let death claim him, but is unable to stop him from transforming. 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's manipulated DNA and he falls to his death to the floor below, returning to his original form and age.
Martha and the Doctor return to her flat, and 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. After they dematerialise, Martha's answering machine records a call from Francine warning Martha about the Doctor, who is unaware that the message came in too late.