Mr. Moto [Peter Lorre] arrives in Puerto Rico to investigate the murder of American agent Graham and uncovers a murderous smuggling ring that stretches into the island’s power structure.
On the ship bound for San Juan, the unlikely alliance forms when the dim-witted but good-hearted professional wrestler Twister McGurk [Warren Hymer] proves to be a capable ally after teaching Moto a few jujutsu moves. Their uneasy partnership blends Moto’s careful deduction with McGurk’s raw strength, creating a dynamic that keeps the criminals off balance.
Accompanying them is Joan Castle [Amanda Duff], the daughter of Colonel Thomas Castle, the island’s steadfast police commissioner, and a political pressure point that complicates every move. Upon landing, Moto feigns appendicitis to be transported to the hospital, a ruse that backfires when the ambulance is hijacked by Captain Dahlen's smugglers who intend to kill him. McGurk arrives just in time to intervene, and Moto, revealing the ruse was deliberate, uses his jujutsu to crush the would-be assassins and regain the upper hand.
Moto discovers that only a few insiders around Governor John Bentley [Paul Harvey] know about Graham’s warning and the alleged appendicitis operation. This suspicion points toward a leader who can move unseen through high circles, while clues from Graham’s library research on a Blackbeard-like pirate named Black Tyrant draw Moto toward a nearby swamp that may conceal the smugglers’ hideout. The plot thickens as another figure, a hotel valet, is killed while attempting to fill Moto’s bathtub rigged for electrocution, underscoring the gang’s reach and ruthlessness.
As the investigation progresses, evidence accumulates suggesting Colonel Thomas Castle’s involvement, and at a glamorous party, a whispered phone call to Captain Dahlen deepens the mystery. Bentley is murdered, and Castle is found at the scene. Commissioner Gordon suspects Castle of the crime, but Moto persuades him to hold off before making any arrests.
When the Castles vanish, Gordon assumes they have fled, but Moto suspects they have been kidnapped. A cable arrives announcing that Moto is actually the master criminal Shimura and that the real Mr. Moto is in New York. Gordon orders the arrest of Moto and McGurk, yet Moto’s mastery of deception and his jujutsu skills enable them to escape. He explains to McGurk that the cable was a ruse designed to infiltrate the gang and locate Joan.
Infiltration leads Moto to a swamp camp where the Castles are indeed being kept, and he is briefly exposed as a double agent when Captain Dahlen arrives. Moto escapes, then informs Gordon of the ruse and leads a coordinated raid. Dahlen escapes with Joan on a boat, but Mr. Sutter shoots Dahlen to prevent him from exposing the truth. Moto, based on medical testimony, predicts Dahlen will reveal his boss at 8 PM. Just before 8, Dahlen is stabbed, and Moto reveals that Colonel Castle was framed all along and that Mr. Sutter is the true leader of the gang, throwing the decisive knife into Dahlen himself to seal the arrest. With Sutter in custody, Joan confesses her feelings for Lt. George Bentley [Robert Lowery], and McGurk thanks Moto for teaching him enough about jujutsu to know he still has much to learn. Moto tactfully tells McGurk that his services won’t be needed on the next case, underscoring the wrestler’s continued learning and growth in the art.
