The Lazarus Experiment

The Doctor returns Martha to her flat, twelve hours relative to after she first stepped in the TARDIS. Clued in by a telephone call by Martha's mother Francine that her sister Tish is on the television, they watch a news report with a statement from the elder Professor Richard Lazarus, announcing that later that night, with one demonstration, he will "change what it means to be human". The Doctor leaves in the TARDIS, but quickly rematerialises when he recognises the impact of Lazarus' statement. They dress for the formal dinner party at Lazarus Labs, and meet up with Tish, who works for Lazarus; Francine, Martha's mother; and Leo, Martha's brother; Francine is immediately suspicious of the Doctor's interest in Martha. Their conversation is interrupted as Lazarus announced he will perform a miracle and steps into a capsule in the center of the reception room. Technicians start the machine, filling the capsule with strange energy, and the Doctor quickly steps in when he believes that the system is overloading. As the machine comes to a stop, Lazarus leaves the capsule, now a much younger man; his machine is able to manipulate the subject's DNA to make them younger. The Doctor, highly concerned of unknown side effects, is able to gain a sample of Lazarus' new DNA when Lazarus kisses Martha's hand, and, with Martha's help, discover that the DNA is unstable.
As they investigate, Lazarus returns to his office along with his partner, the elder Lady Thaw. She insists that she be the next to undergo the process so they can be young together, but he refuses as he didn't want to "waste another life with you [her]". As she threatens to have Mr. Saxon pull their funding, Lazarus begins to undergo a transformation into a scorpion-like being, and kills Lady Thaw by draining her life energy, leaving her a zombie-like corpse; after returning to his human form, Lazarus returns to the reception, while the Doctor and Martha shortly find Thaw's body, the Doctor surmising that Lazarus must drain the life energy to keep his DNA stable. The Doctor and Martha, along with Tish, quickly return to the reception area and warn everyone to escape as Lazarus transforms again. The Doctor lures Lazarus away as Martha and Tish use the sonic screwdriver to override the building's security lockdown and saving the remaining guests. Martha rejoins the Doctor and the two take shelter in Lazarus' device, the Doctor believing he will not harm his own invention. The Doctor also explains that Lazarus's transformation is the result of an evolutionary throwback locked away in dormant human genes that are now becoming dominant. Unfortunately, Lazarus manages to activate his device, but the Doctor reverses the polarity, causing the field to affect only Lazarus outside of the capsule. They emerge to find Lazarus in his human form naked and apparently dead.
As the police and medical workers arrive to take care of the wounded, Lazarus' body is taken by an ambulance. However, when the Doctor hears a nearby crash, he, Martha, and Tish race to discover the ambulance has crashed, its drivers drained of life. They find that Lazarus, alive but human, has taken shelter in Southwark Cathedral. The Doctor tries to reason with Lazarus but is unable to stop him transforming again. Martha and Tish, under the Doctor's orders, lure the creature to the top of the Cathedral's bell tower, while 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 to the floor below, transforming one last time to his human form.
Martha and the Doctor return to her flat, and the Doctor offers Martha on board, more than just as a passenger, which she accepts. As the TARDIS dematerialises from the flat, Martha's voice mail records Francine's warning to her daughter about the dangers of the Doctor as told to her by Mr. Saxon.