Mr. Moto in Danger Island

Moto fakes appendicitis to justify a hospital visit, allowing him to enter the city under medical pretense while the smugglers watch. This ruse reveals who among the insiders knows about the operation, narrowing down suspects to those with access to that information. When the ambulance is hijacked, Moto uses the situation to his advantage, defeating the assassins and exposing the criminals' willingness to kill him.

Mr. Sutter, a wealthy shipper and ally of Governor Bentley, is the true leader of the diamond smuggling ring. He orchestrates the entire operation from behind the scenes, maintaining his respectable public image while directing Captain Dahlen and his criminals. Sutter frames Colonel Castle for Bentley's murder and even kills Dahlen with a throwing knife to prevent him from revealing the truth.

Sutter is present at the library alongside Moto when Graham's research on Black Tyrant is discussed, establishing his early connection to the investigation.

Colonel Castle is framed because Sutter needs a fall guy who has access to the Governor's inner circle. Evidence is planted to suggest Castle's involvement in the smuggling operation, and he is found at the scene of Bentley's murder. This framing allows the real leader, Sutter, to continue operating undetected while the police pursue an innocent man.

Earlier, Joan overhears someone on the phone talking to Captain Dahlen, and evidence mounts suggesting Castle's involvement, all orchestrated by Sutter.

The Shimura cable is a false message sent by Moto himself to orchestrate his infiltration of the gang. The cable falsely announces that Moto is actually the master criminal Shimura and that the real Mr. Moto is in New York. This ruse allows Moto and McGurk to be arrested, then escape and gain access to the smuggling operation as supposed criminals, ultimately leading them to the kidnapped Castles.

The Black Tyrant is a Blackbeard-like pirate from history that Graham researched in the library before his murder. Graham discovered that this pirate used a swamp hideout, which leads Moto to suspect the smugglers are operating from the same swamp area. This historical clue guides Moto to the swamp camp where the criminals are based and where the Castles are eventually held captive.

Captain Dahlen is killed just before 8 PM because he is about to reveal his boss to the police under medical testimony. Mr. Sutter, the true leader, throws a knife into Dahlen to silence him and prevent his identity from being exposed. Sutter's desperation to maintain cover leads him to kill his own subordinate, sealing his guilt and ensuring Moto's deduction that the killer must be someone with authority to move freely.

Moto infiltrates the swamp camp by using the Shimura ruse to gain the criminals' trust. Upon discovering the Castles are being held captive, he is briefly exposed as a double agent when Dahlen arrives. Moto escapes with McGurk's help and contacts Commissioner Gordon to organize a coordinated raid. During the raid, Dahlen escapes with Joan on a boat, but Sutter shoots Dahlen to prevent exposure, allowing Moto to rescue Joan and expose Sutter as the mastermind.

Moto reluctantly teams up with Twister McGurk on the ship to San Juan, initially finding the dim-witted wrestler burdensome. However, McGurk proves invaluable when he saves Moto from the hijacked ambulance attack and teaches him jujutsu, which Moto uses throughout the film to defeat multiple assassins. The partnership combines Moto's deductive skills with McGurk's physical strength, creating an effective team against the criminals.

McGurk proves loyal from the start by saving Moto during the ambulance hijack, establishing his value as an ally.

The real Mr. Moto remains in New York throughout the film. The criminal gang sends a false cable claiming Moto is actually the master criminal Shimura and that the real Mr. Moto is in New York. This fabricated story is part of Moto's own plan to infiltrate the gang - he created the ruse to gain access to the criminals while they believe they have discredited the real detective.

After Commissioner Gordon orders his and McGurk's arrest based on the false Shimura cable, Moto reveals the cable was his own ruse to infiltrate the gang. Using his jujutsu mastery, Moto and McGurk escape from police custody, allowing Moto to continue his investigation and locate the kidnapped Castles in the swamp camp.