The TARDIS lands on Fang Rock island at the start of the 20th century. There the Doctor and Leela find that a lighthouse keeper has been killed and a creature is at large...
A thick fog surrounds Fang Rock and a ship is wrecked. The survivors take shelter in the lighthouse but all are in deadly peril from the alien menace that stalks the island.
The Doctor seals the survivors inside the lighthouse but soon realises that he might have trapped the creature inside with them by accident.
As the alien Rutan picks off the remaining humans one by one, the Doctor devises a desperate plan to defeat the alien and repel the approaching Rutan mothership.
