Professor Richard Lazarus was an elderly scientist and businessman in 21st century London. As a boy, he had survived the Blitz by hiding inside of a crypt. Since then, he became fearful of death and after becoming a scientist began experiments in age regression, through funding provided by Lady Sylvia Thaw and her mysterious benefactor Harold Saxon. He eventually succeeded by the time he was 76, and, in front of a rapt audience underwent an age regression process inside of a chamber that returned him to his early thirties.
However, the process had an unexpected side effect, unlocking dormant DNA within Lazarus' body. He quickly began transforming into a prehistoric monster, a creature that the Doctor decided was a path of evolution humanity could've potentially taken but didn't. In order to stave off the transformations, and maintain his youth, Lazarus had to feed off the life energy of other people. His first victim was Lady Thaw. Afetrwards, he lured Tish Jones to the roof of his building, but was interrupted in his attempt to kill her by the Doctor and Martha.
Transforming fully, Lazarus chased them downstairs to his own party where he killed at least one woman before the Doctor lured him away so everybody could escape. The Doctor rewired the de-aging chamber to affect things outside of it, instead of inside, returning Lazarus to his human form. He was taken to the hospital but en route escaped, killed the paramedics, and fled to a nearby cathedral. The Doctor, Martha and Tish followed him here. He corned Martha and Tish in the belltower, but the Doctor used his sonic screwdriver to amplify the sound of the church's pipe organ, and, playing it, disoriented Lazarus so he fell down the tower to his death.