During the Time War, the Eighth Doctor attempts to rescue a pilot, Cass, whose spacecraft is crashing into the planet Karn. When Cass realizes that the Doctor is a Time Lord, she refuses his aid, ignoring his claims that he has never taken part in the devastating Time War. The Doctor refuses to abandon Cass, and both are killed when the ship crashes.
On the planet, the Doctor is taken in by the Sisterhood of Karn, guardians of the Flame and Elixir of Eternal Life, who revive him temporarily; Cass, however, is beyond their help. The Sisterhood offer the Doctor a selection of potions which, if consumed before he expires, will trigger his regeneration into a new form. They convince the Doctor that he must take action to end the Time War, which “threatens all reality”. The Doctor’s initial rejection is met by the response that he is “dead already; how many more will you let join you?”, and the Sisters beg him not to allow the universe to fall, as it presently “stands on the brink”. The Doctor acknowledges there is not much need for a doctor any more, salutes the memory of his past companions, and asks for a potion that will turn him into “a warrior”. He utters his last words (‘Physician, heal thyself’) and, after drinking it, regenerates into an new incarnation, known as the War Doctor, who takes Cass’s bandolier and declares, “Doctor no more.”