Leo Grainger, a former investment broker recently released from prison, now lives with his stepdaughter Gail on Fog Island, once a refuge for pirates. Gail has fled to the island because Leo's release has prompted renewed newspaper interest in the story of his supposed investment swindle and Gail's mother Karma's violent death during his imprisonment. Aware that his former associates, who invested their money foolishly and then accused him of stealing their savings, believe that he has hidden a fortune somewhere on Fog Island, Leo announces to Gail that he has invited them all for the weekend. Leo is confident that the greedy group-Alec Ritchfield, John Kavanaugh, Sylvia Jordan, Emiline Bronson and Kingsley-will be lured by his promise of just retribution and will not suspect that he is setting a death trap for them. As predicted, Alec, John, Sylvia and Emiline, a fortune-teller, take a launch to the island, as does Kingsley's son Jeff, who is coming in his now-deceased father's stead. Also present is Dr. Lake, Leo's sinister ex-accountant, who served time with him. While Jeff tries to reingratiate himself with the seemingly indifferent Gail, who until recently was his girl friend, Alec, John, Sylvia and Emiline accept token gifts from their host. As soon as Leo retires for the night, Sylvia and John plot to steal the key that Leo gave Gail as her token, while Emiline discovers a note inside her gift, which directs her to look in a box on the mantlepiece. Before Emiline can open the box, however, Alec persuades her to conduct a se? nce around a large desk. During the se? nce, Alec slips away and enters the house's secret passageway, the door to which opens when a certain stop on the mansion's old organ is pulled. The se? nce ends abruptly when the desk starts to levitate, and the guests retire to their rooms. Feigning friendliness, Sylvia tries to steal the key from Gail, but Emiline interrupts her. Lake, meanwhile, accosts Allerton, Leo's butler, and after identifying him as escaped convict Al Jenks, throws him out a window to the churning sea below. Later, Alec sneaks down to retrieve Emiline's still unclaimed key but is interrupted by Leo, who accuses him of killing Karma after she refused to reveal the location of his fortune. Alec stabs and kills Leo, then drags his body into the passageway and stuffs it into a vault in the cellar floor. Alec returns to the mantle just as Emilene finds the key and offers to show her where it fits in exchange for half of Leo's booty. As Alec is leading Emiline into the passageway, however, he stabs her. Meanwhile, upstairs, Jeff, who has been unsuccessful at getting Gail to open up, finds Lake rifling through a desk, and the two start to fight. Lake knocks out Jeff and flees, and the noise of the struggle awakens Gail. Then, while Gail and a revived Jeff try to open a secret drawer in the desk, about which her mother had once told her, Sylvia and John meet up with Alec in the passageway. Using all the clues from their tokens, the three make their way to the cavernous cellar and, after being joined by Lake, locate Leo's treasure box, which they discover contains only a spiteful note. The cellar's entrance then closes and sea water begins seeping in through the floor. Gail and Jeff, meanwhile, activate a trigger in the desk, which causes it to levitate and reveal a hidden drawer. Gail opens the drawer with Leo's key and finds her mother's jewels and a note, in which her mother advises her to forget about the past and move on with her life. After admitting to Jeff that she ran away because she feared that others suspected her of trying to steal Leo's fortune, Gail prepares to leave the island with Jeff. While readying to go, Jeff stumbles onto the passageway and discovers not only Leo's body but the drowned Alec, Sylvia, John and Lake as well. Without telling Gail the horrors he was witnessed, Jeff escorts Gail to the waiting launch.
