For seven years, opera star Gravelle (Boris Karloff) has been locked in an insane asylum, his identity a mystery – even to himself.

But when his memory unexpectedly returns, he begins to recall that his wife and her lover tried to murder him – and now he's determined to make them face the music.

Gravelle escapes from the asylum and makes his way to the San Marco opera house and begins hiding out in the various rooms and passageways.

The suspects, excluding Gravelle, include Lilli Rochelle, the opera company's prima donna who has been having a secret affair with Enrico Barelli, the baritone; Mr. Whitely, Madame Rochelle's husband who has warned Barelli to stay away from his wife; Anita Barelli, the opera company's number two soprano who has learned of her husband's affair with Lilli Rochelle; and Phil Childers, the fiancée of Lilli's unacknowledged daughter who has been refused permission to marry the daughter.

Among the questions asked are who has been threatening Lilli Rochelle's life, the mystery man in Barelli's dressing room before he is murdered, and why does Chan insist that the opera be performed twice in one evening?