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In the third episode of the four-part story "The Invisible Enemy," the miniaturized clones of the Doctor (Tom Baker), Leela (Louise Jameson), and robotic dog K-9 continue their "fantastic journey" through the real Doctor's blood stream, endeavoring to locate and destroy the infectuous space virus called the Swarm. Alas, the virus manages to escape the Doctor's body and grows to full size, the better to replicate itself many times over -- and then to take over the Universe. Written by Bob Baker and David Martin, "The Invisible Enemy, Episode 3" originally aired on October 15, 1977.