- Episodes: 4 Episodes
- Doctor: 4th Doctor
- Companions: Sarah-Jane, Harry, Lethbridge-Stewart, Benton
- Aliens/Monsters: K1 Robot
- Setting: Earth
With the newly-regenerated Doctor (Tom Baker) behaving erratically, Brigadier Lethbridge-Stewart and UNIT have to contend with the theft of the plans for the new disintegrator gun by what seems to be a mechanical monster. However, the theft is only the beginning of a plot to hold the world to nuclear ransom.