Doctor Who: The Power of Kroll, Episode 4 (1979)

In the conclusion of the four-part story "The Power of Kroll," the Doctor (Tom Baker) and Romana (Mary Tamm) are forced to overcome enormous obstacles in their efforts to retrieve the fifth segment of the Key to Time on the swamp moon of Delta Magna. But the cream of the jest occurs at the climax, when the Doctor discovers that the Great God Kroll, whom the local Swampies worship blindly, is actually a huge squid -- and Kroll is, in fact, the elusive Fifth Segment. Written by Robert Holmes, "The Power of Kroll, Episode 4" originally aired on January 13, 1979. ~ Hal Erickson, All Movie Guide