Determined to find his girl friend Alice, who disappeared a year before in Chicago, Tim Winthrop goes to New York and seeks the help of his friend, criminal lawyer Clay Dalzell. After discussing the situation, Tim, Clay and Donna Mantin, Clay's would-be fiancée, go to the theater, where Mary Smith, an actress who is famous for the lifelike mask that she wears on stage, is performing. As the curtain rises, Clay receives a message to meet gangster Jimmy Kinland and leaves to pick up a packet of incriminating love letters that Donna has asked him to retrieve. While Clay is negotiating with Kinland for the letters, which he discovers are not from Donna, but from a married friend, he hears a radio news report about the sudden disappearance of Mary Smith during the play's first act. Clay returns home and is visited first by Tim, who tells him that Mary Smith is Alice, and then by newspaper gossip columnist Tommy Tennant. Before Tommy can explain why Mary Smith bolted from the stage, he is shot and killed by an unseen gunman in Clay's bedroom, who also grazes Clay. Before escaping, the killer throws his gun next to Clay, thereby incriminating the lawyer in Tommy's murder. Now the main suspect of Inspector Doremus, Clay determines to find the killer and, with the help of Donna and Horace Swayne, his butler, undertakes to connect the woman's disappearance with Tennant's murder. Eventually Clay finds out that his former lover, Jerry Classon, and her wealthy lawyer husband Roger are also looking for Alice, who Classon claims is the only person who can exonerate his partner in the murder of a Chicago mobster. From Kinland, Clay learns that Alice's father had been ruined by John Moroni, Classon's partner, and that, out of revenge, she fled Chicago to avoid providing him with an alibi, and then became Mary Smith. From Donna, Clay hears that Jerry had affairs with both Moroni and the slain gangster. Sure that the killer's target is Alice, Clay telephones all of his suspects and tells them that Mary will be waiting for them at a certain Greenwich Village address. By using a recording of Mary's singing, Clay, Donna and Doremus trap the killer, who turns out to be Classon dressed in women's clothes and Mary's mask. As deduced by Clay, Classon killed the Chicago gangster out of jealousy and, wanting to pin the crime on the equally guilty Moroni, tried to eliminate Alice as Moroni's alibi. The murder solved, Tim and Alice reunite and marry on the same day that Donna finally snags the elusive Clay.