In Usher, his great country house, aging horror star Romney Marsh gathers a group to read a new play. When the last guest arrives, Romney unleashes the guard dogs and, with a moat full of piranha, he has his visitors trapped. Someone has been sending him newspaper clippings about the suicide thirty years ago of Mabel Monk, an actress and Romney's lover. He intends to find out who is responsible, but first one of his quests is found with her throat cut. Then Romney dies, poisoned by his own chocolates. Marooned in the house with thick fog swirling outside, the guests realize that the murderer must have some connection with Mabel. The finger of suspicion points at each of them until the truth is revealed with fatal consequences.