Synopsis
The Doctor and Rose Tyler are preparing to enjoy a visit to Earth's Moon when they discover that the TARDIS has, on its own volition, taken them instead to a small seaside town in present-day England. In this town, the dead shell of a Dalek has been discovered by archaeologists in 1st century Roman ruins on the site of a decommissioned Cold War-era military bunker.
The Doctor and Rose are separated for a time when the TARDIS dematerializes with Rose still outside the ship. The Doctor finds himself at the dig where he befriends one of the archaeologists and, upon recognizing the Dalek, attempts to disarm the dead creature by giving his new friend custody of the Dalek's gun arm.
Rose, meanwhile, witnesses a traffic accident in which a young woman named Kate is apparently killed. However, Kate proceeds to regenerate in much the same fashion as the Doctor, which causes her hair colour to change and — unknown to Rose — her intellect to increase exponentially. Along with Kate's newfound intelligence comes a confusing desire to exterminate every human on the planet, starting with her ex-boyfriend.
The Doctor and Rose eventually reunite at the dig site, where Kate reactivates the Dalek, causing it to also regenerate. Rose and the confused Kate escape, while the Doctor tries to unsuccessfully disable the revived Dalek before it can go on a killing spree. The Dalek tracks down and kills the archeologist in order to reacquire its gun arm.
Meanwhile, Kate's personality becomes more Dalek-like, and she eventually unites with the Dalek, who bargains for the use of a time travel technology, with Earth as the bargaining chip.
It is revealed that Kate is a form of Dalek-human hybrid, the result of an attempt by Daleks who came to Earth millennia earlier (because of the Time War) and injected a "Dalek Factor" into humanity. However, only a few individuals retained it in the present day.
With encouragement from Rose and the Doctor, Kate's human personality manages to reassert itself against the Dalek influence and she destroys the Dalek, returning to her pre-accident self.
Read more about this topic: I Am A Dalek