Doctor Who: Planet of the Daleks, Episode 4 (1973)

In the fourth episode of the six-part story "Planet of the Daleks," the Daleks have enslaved the inhabitants of the planet Spiridon, in hopes of learning their captors' secret of invisibility. Meanwhile, the Doctor (Jon Pertwee), Jo (Katy Manning), and a group of Thals step up their efforts to prevent the Daleks from conquering the universe. The highlight of this episode is a terrifying evening in the Spiridon jungle. Written by Terry Nation, "Planet of the Daleks, Episode 4" first aired on April 28, 1973. ~ Hal Erickson, All Movie Guide