20. 'Oh, So You're My Replacements. A Dandy and a Clown.'
From: "The Three Doctors"
Premiere: December 30, 1972
Given the time-traipsing premise of Doctor Who, it was practically preordained that one day a given Doctor might meet another of his incarnations. It happened first in 1972, in a serial that marked the tenth anniversary of the series. Confronted with the menace of insane Gallifreyan forefather Omega (Stephen Thorne), the Time Lords allow the Third Doctor to work in concert with his two predecessors. Watching Pertwee and Troughton battle for the spotlight is more fun than the actual story; best of all is the withering assessment of the pair delivered by a visibly infirm but feisty Hartnell.