It is the turn of the century, and the Tardis materialses by the lighthouse on the desolate isle of Fang Rock. When the lighthouse engineer dies in mysterious circumstances, the remaining crew blame the mythical Beast of Fang Rock; that is, until the Doctor and Leela turn up. When a small clipper runs aground on the crags of Fang Rock, the lighthouse offers shelter to its despairing passengers. But the Doctor soon discovers clues that suggest that no one is safe on the tiny island.
Whilst a private drama unfolds between the survivors of the wrecked clipper, another death occurs. The Doctor suspects a ruthless alien lurks in the thick unnatural fog around the island, and secures the lighthouse. But has he really locked the horror out, or is it somewhere inside with them?
Episodes in this story originally transmitted on BBC1 03/09/77 - 24/03/78.