Mr. Moto’s Last Warning is the sixth in a series of eight films starring Peter Lorre as Mr. Moto.

The film is an original story featuring the character created by John P. Marquand.

Plot
The British Navy in Port Said is making plans for naval manoeuvres with the French fleet. Plans are delayed because the British Secret Service has been warned of possible sabotage. On a ship docking in Port Said is Madame Delacour (Margaret Irving), wife of the French naval admiral. Delacour and her daughter Marie (Joan Carroll) are befriended by the charming Eric Norvel (George Sanders), the goofy Rollo Venables (Robert Coote), and someone posing as Mr. Moto (Teru Shimada). Norvel reveals his true nature when the ship docks and he lures Mr. Moto to his death. This “fake” Mr. Moto turns out to be a fellow agent of Mr. Moto (Peter Lorre) who is posing as a curio dealer Mr. Kuroki.

Norvel takes Delacour and Venables to a variety show featuring Fabian the Great (Ricardo Cortez), a ventriloquist. Fabian is the leader of the gang of saboteurs including Hakim (John Davidson), Captain Hawkins (Leyland Hodgson), Danforth (John Carradine), and Norvel. Danforth is actually a British Secret Service agent named Burke. Moto listens in on their conversation and is almost captured, but Burke helps him escape. Norvel is given the task of learning from Delacour when the French fleet is due at Port Said.

The suspicious Fabian thinks that Kuroki may actually be Mr. Moto. Fabian enlists his girlfriend Connie (Virginia Field), who is unaware he is an agent, to follow Mr. Moto the next day. She sees Moto visit the Port Commandant’s office where he learns of the salvage ship “The Vulcan” captained by Hawkins.

Fabian also discovers that Danforth is the secret agent, Burke. Fabian lures Burke to “The Vulcan” and reveals his plan to blow up the French fleet and blame the British. He then kills Burke by trapping him in a diving bell. Norvel gets the information needed and tells Fabian at the theater. Connie overhears the conversation and threatens to call the Port Commandant, but Fabian convinces her to go along with him.

Hakim tries to kill Moto with a bomb, but Moto escapes the explosion and follows Hakim to a warehouse. Moto enlists Venables to help, but Venables is tricked by Norvel. After a fight, Moto and Venables are tied in sacks and thrown into the ocean, but not before Moto tricks Hawkins and grabs a piece of sharp metal. Connie can’t bear to see this violence and goes to call the police. Fabian knocks her out and proceeds with his plans.

Moto escapes underwater and frees Venables, who goes to the police. Norvel dives down to await a signal from Fabian but Moto overpowers him and prematurely detonates the explosives meant to destroy the French fleet. Resurfacing, Moto fights with Fabian but Connie shows up and shoots Fabian. Moto discovers the saboteur’s plans in Fabian’s dummy but never reveals to the audience which country tried to engineer a war between France and England.

Cast
Peter Lorre as Mr. Kentaro Moto (a.k.a. Mr. Kiroki)
Ricardo Cortez as Fabian the Great
Virginia Field as Connie Porter
John Carradine as Danforth (a.k.a. Richard Burke)
George Sanders as Eric Norvel
Joan Carroll as Mary “Marie” Delacour (credited as Joan Carol)
Robert Coote as Rollo Venables
Margaret Irving as Madame Delacour
Leyland Hodgson as Captain Bert Hawkins
John Davidson as Hakim
Teru Shimada as the Fake Mr. Moto