Doctor Who: The Invisible Enemy, Episode 1 (1977)

Synopsis
The "Invisible Enemy" of the title is an intelligent space virus called the Swarm, which the crew of an earth base on the Jupiterian moon Titan. In his efforts to stop the Swarm, the Doctor (Tom Baker) is himself infected -- leading to a truly "fantastic voyage" for the miniaturized clones of the Doctor and Leela (Louise Jameson), not to mention a brand-new edition to the Doctor Who supporting cast. Written by Bob Baker and David Martin, "The Invisible Enemy" began its four-episode run on October 1, 1977. ~ Hal Erickson, All Movie Guide