"The Lazarus Experiment" is the sixth episode of the third series of the revived British science fiction television series Doctor Who. It was broadcast on BBC One on 5 May 2007 and stars David Tennant as the Tenth Doctor and Martha Jones as his companion, played by Freema Agyeman. On Earth, in present-day London, The Doctor, Martha and her family attend a scientific demonstration by the aged Professor Lazarus. But when the experiment goes wrong, a horrific product of genetic manipulation is unleashed, leaving a trail of death in its mission to live indefinitely.

According to the BARB figures this episode was seen by 7.19 million viewers and was the twelfth most popular broadcast on British television in that week. Executive producer Russell T Davies has stated that he directed writer Stephen Greenhorn to base this episode on the typical Marvel Comics plotline: "a good old mad scientist, with an experiment gone wrong, and an outrageous supervillain on the loose."

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.

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