Celebrated as supersleuth, Mr. Moto comes out fighting when a brutal boxing match turns into cold-blooded murder! Assisted by detective-in-training Lee Chan, Moto sets out to track down the killer based on a single ominous clue: a poisoned boxing glove! But when Moto's hunch points to a corrupt gambling syndicate, he's forced to wager his very life to unmask the culprit—or go down for the count...permanently!

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.

Allen Colby, heir to a huge fortune, is presumed drowned after an ocean liner sinks off the coast of Honolulu. Mysteriously, Colby reappears at his mansion only to be murdered soon after. When his body is discovered during a seance, everyone in attendance becomes a suspect, and it's up to Chan to find the murderer before he or she strikes again.