3. The First Doctor Regenerates
From: "The Tenth Planet"
Premiere: October 8, 1966
"It's far from being all over." With those words, William Hartnell's First Doctor shuffled toward his shocking demise – and transformed into the Second Doctor, played by Patrick Troughton. Were it only for the introduction of the Cybermen – a weird cyborg race from Earth's wayward twin planet, Mondas, and another of the Doctor's most durable nemeses – then William Hartnell's swansong in the title role would still merit iconic status. But "The Tenth Planet" also ushered in a new facet to the Doctor's alien biology, his race's ability to regenerate into a completely new form – assuring that Doctor Who could carry on even without the actor who played the title role.