In the conclusion of the six-part story "The Invasion of Time," the Sontarans launch their invasion of the Time Lords' home world of Gallifrey. As Lord President, the Doctor (
Tom Baker) tackles the responsibility of fending off the Sontarans. But first, he must assemble the forbidden de-mat gun, controlled by the Great Key of Rassilon -- and this act has a very profound effect on both the Doctor and his traveling companion, Leila (Louise Jameson, making her farewell
Doctor Who appearance). Written by Graham Williams and Anthony Read under the joint pen name of David Agnew, "The Invasion of Time, Episode 6" originally aired on March 11, 1978, as the final episode of
Doctor Who's 15th season. ~ Hal Erickson, All Movie Guide