The Fourth Doctor (Tom Baker) visits Earth circa 10,340 A.D. with Sarah Jane and Harry, expecting to find it deserted. They discover a small group of humans--a military expedition that came because of a distress signal. The group is very suspicious of the Doctor and his companions since no one had been living there for thousands of years and a mysterious "rock creature" has been kidnapping and torturing them. The creature turns out to be a Sontaran who has been doing some research on the humans in preparation for a galactic invasion.
The story is a typical cat-and-mouse game between the Doctor's set and the lone Sontaran. The Sontaran's military creed and ruthlessness should make him more threatening but he's not all that impressive in this episode. Some good comic lines are delivered by the Doctor and the Sontaran, but not enough to make the show a fun comedy. This two episode story is happily short because the story just doesn't have enough to keep up the dramatic tension.
This show is for Doctor Who completists. The DVD also includes a nice special feature on the history of the Sontarans in the classic Doctor Who years.