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In the second episode of the four-part story "The Invisible Enemy," the Doctor (Tom Baker) has been infected by an intelligent space virus called the Swarm. At the Bi-Al foundation, miniaturized clones of the Doctor, Leela (Louise Jameson), and robotic dog K-9 are injected into the real Doctor's bloodstream, in hopes of locating and destroying the virus before it can succeed in its takeover of the Universe. Obviously inspired by the 1966 theatrical feature Fantastic Voyage, "The Invisible Enemy, Episode 2" was written by Bob Baker and David Martin, and originally aired on October 8, 1977.