
In Puerto Rico to investigate a glut of contraband diamonds that are flooding the world's jewel market, Mr. Moto and his sidekick, a wrestler, find themselves involved in murders by thrown daggers, the frame-up of an overstressed Army colonel, and a pirate gang led by an unknown boss who has inside knowledge of the ensuing investigation.

Peter Lorre
Kentaro Moto

Warren Hymer
Twister McGurk

Amanda Duff
Joan Castle

Robert Lowery
Lt. George Bentley

Leon Ames
Commissioner Madero

Jean Hersholt
Mr. Sutter

Douglass Dumbrille
Comdr. La Costa

Richard Lane
Police Commissioner Gordon

Charles D. Brown
Capt. Thomas Castle

Paul Harvey
Gov. John Bentley

Eddie Marr
Capt. Dahlen

Harry Woods
Grant

Ward Bond
Sailor Sam - Wrestler

Neely Edwards
Moore - Governor's Aide

Renie Riano
Librarian

Willie Best
Launch Pilot

Grace Hayle
Mrs. Brown

Edwin Stanley
Doctor at Hospital
Walter Deming
Drunk

Lester Dorr
Real Ambulance Intern
Herbert I. Leeds
Peter Milne
Released