Mr Moto (Peter Lorre) heads to Puerto Rico to uncover a diamond smuggling gang, on the way he befriends a pro wrestler Twister (Warren Hymer) who ends up assisting Moto.
Someone high up on the island is behind the diamond smuggling plot and tries to get rid of Moto even more he can begin his investigation. Moto disguises himself as a Japanese criminal in order to try and infiltrate the gang and uncover the mastermind...
A good crime B-move with the usual fast moving action, well structured plot and red herrings. Interestingly, this was originally to be a Charlie Chan film set in Trinidad but changed to become a Moto story after the death of Warner Oland.
