A three-man shuttle crew are completing the final stages of their journey to Titan base, when a course deviation puts the shuttle in the path of a mysterious cloud in space. The crew members arrive on Titan, but they have been somehow changed, infected by an intelligent virus that threatens everyone in the galaxy. The TARDIS picks up a Mayday message from Titan base, but before the Doctor and Leela can fully respond, the police box also passes through the mysterious cloud. With the infection spreading from person to person, the Doctor and Leela seek assistance on a medical station in the asteroid belt. As the situation worsens, their only hope lies in the unlikely combination of the eccentric scientist Professor Marius, and his dog-shaped computer K9...
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