23. Cybermen March Down From St. Paul's Cathedral
From: "The Invasion"
Premiere: November 2, 1968
As when the Daleks rolled through London on the First Doctor's watch, this milestone serial offered the image of militaristic ranks of invading Cybermen tromping down the steps in front of St. Paul's Cathedral. The story also reintroduced one of the most durable secondary characters in the series: Brigadier Lethbridge-Stewart (Nicholas Courtney), previously encountered as a colonel in "The Web of Fear," and now charged with operating UNIT, a military task force that will play a significant role in the Third Doctor's earthbound adventures.