While on a holiday at a 23rd-century recreation planet, the Doctor encounters a grisly accident and murder.

The Doctor follows a mysterious creature attacking Scottish oil rigs to Loch Ness where not only does he discover that the legendary monster does exist, but that is also a weapon of the Zygons in their plan to overpower the planet.

The Doctor and Vicki are surprised to discover a stow-away aboard the TARDIS and welcome Steven Taylor, who survived the destruction of the Mechonoid city, aboard.

A newly regenerated Doctor joins UNIT in an investigation into the theft of top secret plans and equipment from supposedly secure premises. Sarah discovers that the raids have been carried out by a robot invented by scientist Professor Kettlewell while he was working for Think Tank, a body involved in developing emerging technologies.

The Earthlings prepare to return to their deserted planet but their plans are thwarted by the arrival of a curious Sontaran.

Field Major Styre chooses the Doctor as his opponent in a duel to the death.