The Invisible Enemy

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...

• Long time contributing guest star Michael Sheard returns (this time as Lowe).

• This story introduces one of the most popular companions from Doctor Who, K-9.  The early model of K-9 proved to be very troublesome.  When it got close to a camera the picture would be distorted and the radio controlled K-9 would no longer follow commands from the control panel.  It would be some time before these problems could be overcome.  As a result, K-9 was used sporadically during the rest of the season.

• John Leeson provides the voice of K-9 as well as the voice of the swarm leader.

• Another actor returning for a guest role is Frederick Jaeger as Professor Marius.  Jaeger also appeared two years earlier as Dr. Sorenson in "Planet of Evil" and as Jano in the Hartnell story "The Savages."

• "The Invisible Enemy" was an ambitious story from a production design viewpoint as the story concept borrows heavily from the sci-fi classic "Fantastic Voyage."

• Playing Lowe is none other than Michael Sheard.  Sheard's first role in Doctor Who was in the 1966 Hartnell story "The Ark" as Rhos.  5 years later Sheard would return as Dr. Summers in the Pertwee story "The Mind of Evil."  Sheard would again appear in Doctor Who, this time as Lawrence Scarman in the classic Tom Baker story "Pyramids of Mars."  After his appearance in "The Invisible Enemy" he took a 4 year break from Doctor Who until his next appearance as Mergrage in "Castrovalva."  One final appearance occurred in the 1988 Sylvester McCoy story "Remembrance of the Daleks."  He's a busy man...