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Cyrus Townsend, father of Alice, is mysteriously slain in his home and suspicion centers on Arthur Marsden, his secretary and Alice's sweetheart. The circumstances of the murder are similar to those of numerous unsolved killings believed to have been perpetrated by a gorilla. As Alice, Marsden, and Stevens (a friend of Cyrus) are gathered in the library, a note warns them to leave the house before midnight. Later, they are terrified by the unexpected arrival of Mulligan and Garrity, two callous detectives sent to solve the mystery; subsequently, persons suddenly vanish, strange noises are heard, and lights go off and on. While searching for each other, the detectives discover a gorilla on the premises; a sailor confesses to being the gorilla; but Marsden, who reveals himself also to be a detective, leads them to Stevens, the true culprit.