In the conclusion of the four-part story "The Ark in Space," the Doctor (Tom Baker) and his friends struggle to save the last remaining vestiges of Mankind from being devoured and absorbed by the insectasoid Wirrrns. But will the Doctor's efforts spell the doom for the Nerva Beacon space station? The conclusion of this episode neatly segues into the next Doctor Who continuity, "The Sontaran Experiment." Written by Robert Holmes, "The Ark in Space, Episode 4" originally aired on February 15, 1975. ~ Hal Erickson, Rovi