In which story did the Doctor first regenerate on screen?

The Doctor first regenerated on screen in the Doctor Who story “The Tenth Planet.” At the end of the 1966 serial, William Hartnell’s First Doctor collapses inside the TARDIS and transforms into Patrick Troughton’s Second Doctor.

The story does not yet use the later, familiar term “regeneration.” The Doctor describes the process as a renewal or change of appearance, and the mythology surrounding regeneration was developed more fully in later stories. Nevertheless, “The Tenth Planet” established the central device that allowed the programme to continue with a new lead actor.

The First Doctor is weakened after confronting the Cybermen and the energy-draining effects of Mondas, Earth’s former twin planet. After returning to the TARDIS, his body begins to change, creating the first transition between actors playing the Doctor.

“The Tenth Planet” was broadcast in four parts from 8 to 29 October 1966. Its fourth episode is largely missing from the BBC archive, although surviving footage, audio, telesnaps, and later animation preserve the historic regeneration sequence. The story is also notable as the Cybermen’s first appearance.