Based on a novel by Stuart Palmer, an amateur detective and a New York police inspector solve two murders at Catalina island.
A sea plane flies to Catalina, and Roswell Forrest feels sick; when they land, he is found dead. Hildegarde Withers (Edna May Oliver) goes to the police chief; but a doctor in a swim suit calls it heart failure. Hildegarde suspects poison. The police chief reads an unopened letter to Forrest warning him. In New York inspector Oscar Piper (James Gleason) gets a telegram from Hildegarde asking for information on Forrest, who is a key witness against the Graver mob. Piper goes to protect Hildegarde.
Passengers saw movie-producer Joseph Tate (Leo G. Carroll) give Forrest a drink. Hildegarde says the cigarette was poisoned. Hildegarde overhears Tate giving Phyllis La Font (Lola Lane) a part in a movie to keep her mouth shut. Piper and Hildegarde go to check the autopsy, and the doctor discovers the body is gone. They find a footprint. Piper accuses Marvin Deving (Harry Ellerbe), but many have that footprint. Phyllis tells Hildegarde that Tate's flask has two compartments. Mrs. Kay Deving (Dorothy Libaire) saw the pilots moving a body to the airport; but they say they took Tate for $25. Tate explains his flask and why he wanted to get away.
Hildegarde sees Arthur Mack pick up an envelope at the hotel desk and hears his mailbox combination when he is on the phone. She follows him and takes out the envelope he put in, finding $10,000. She hires the porter Willie to watch the mailbox. Piper found the wheelbarrow at the casino and calls Hildegarde there. They find Beegle drunk and the Devings. Marvin Deving is shot dead. Piper calls the police and arrests Beegle, but he has an alibi. Hildegarde and Piper find the buried body. Piper arrests Tom Kelsey (George Meeker), who is really Forrest; he says the man killed was his bodyguard Kelsey pretending to be him. Hildegarde finds dead fish in a pond with the cigarettes. Arthur Mack with a gun finds her in his room and ties her up. She wakes up with Piper, and they find Mack dead. She offers Mrs. Deving a cigarette, exposing her when she refuses to smoke it. She pulls a gun; but Piper is able to get it away from her. Arthur killed Deving to save himself for not paying the money, and the Devings thought they killed Forrest. The real Forrest leaves the evidence against the mob for Piper and has run off with Phyllis, who thanks Hildegarde and leaves her her dog with pups.
In this amusing mystery Piper keeps arresting the wrong person while he competes with Hildegarde, who commits a federal offense by robbing $10,000 from a mailbox, thus helping to cause one of the murders.