One goes and another comes in the Christmas episode of "Doctor Who". Matt Smith left the sci-fi series in the special titled "The Time of the Doctor", which introduced Peter Capaldi as the new version of the Doctor.
[SPOILER ALERT!] After saving his companion Clara from a family dinner, the Doctor and his assistant got transported to a faraway land where clothes were banned. Smith delivered one jaw-dropping scene as he got naked, but the funny moment soon ended as the Doctor learned that he's the 13th (last) incarnation in this cycle, meaning he that he would vanish forever when he died.
The Doctor was seen aging and fighting his enemies on his own for one last time after dropping Clara back on Earth. When he's dying, Tasha, the head of the Church of the Papal Mainframe, picked Clara up on Earth to accompany the Doctor in his final moments.
The Doctor was then transformed into a new version of himself, played by Capaldi, who's also the first Doctor in the new cycle.