Mr. Moto in Danger Island

Mr. Moto arrives in Puerto Rico to investigate the murder of American agent Graham and begins to trace a murderous diamond-smuggling ring. On the ship to San Juan, he reluctantly gains the help of Twister McGurk, a dim-witted but good-hearted wrestler, who teaches him jujutsu.

During the voyage, McGurk trains Moto in basic jujutsu and becomes a loyal, if unconventional, ally. Moto quickly recognizes McGurk's practical combat skills and the pair begin to work as a team.

Upon landing, Moto pretends to suffer appendicitis to justify a hospital visit, but the ambulance is hijacked by the smugglers led by Captain Dahlen. McGurk arrives in time to save him, and Moto reveals he was never actually ill, using his jujutsu to defeat the would-be assassins.

Moto realizes that only a few people know about the operation, so he deduces that one of them must be the smuggling ring leader. This insight narrows the suspects and moves the investigation forward.

At the library, Mr. Sutter, a wealthy shipper and governor's ally, is present alongside Moto. Graham's library research on a Blackbeard-like pirate known as Black Tyrant and his rumored swamp hideout leads Moto to suspect the smugglers are using the nearby swamp as their base.

Moto and McGurk are fired upon when they visit the swamp camp, confirming the gang's stronghold and their willingness to attack outsiders. The encounter escalates the pursuit and tightens the mystery around the leaders.

Back in San Juan, a hotel valet is killed while trying to fill Moto's bathtub, a trap that hints at the criminals' reach. Later, evidence emerges that Colonel Castle is involved in the smuggling, and Joan Castle overhears someone on the phone talking to Captain Dahlen.

At a party, Bentley is murdered and Colonel Castle is found at the crime scene, sparking a feud with Commissioner Gordon who accuses Castle of the murder. Moto persuades Gordon to withhold judgment and continue the investigation.

The next day, Gordon, Moto and McGurk go to Castle's home and find the Castles have disappeared, leading Gordon to assume they were kidnapped or fled. He orders Castle's arrest while Moto urges caution.

A cable arrives announcing that Moto is the master criminal Shimura and that the real Moto is in New York, prompting Gordon to arrest Moto and McGurk. Moto then reveals the cable was a ruse to infiltrate the smuggling gang.

Moto infiltrates the gang camp in the swamp and discovers the Castles have been kidnapped. He is exposed as a double agent when Dahlen arrives, but he escapes with McGurk and contacts Gordon to organize a raid.

During the raid, Dahlen is taken to the hospital and declares he will reveal his boss at 8 pm. Just before 8 pm, Dahlen is stabbed, and Moto reveals that Colonel Castle has been framed and the real leader is Mr. Sutter, who is caught throwing the knife into Dahlen. Sutter is arrested, and Joan declares her love for George Bentley as Moto hints that McGurk still has much to learn.