The film opens with Mr. Moto in disguise as a street salesmen and selling goods to passers-by. He sees a man leaving a shop with a tattoo of the British Flag on his arm. Moto enters the shop to sell a rare diamond to the owner. However, Moto sees a body stuffed into a wicker basket in the store, and using his mastery of judo takes down the shopkeeper. Later, he reserves a berth on a cargo ship headed for Shanghai. Also on the freighter is Bob Hitchings Jr., son of the owner of the freighter. Before leaving, Hitchings Sr. gives his son a confidential letter for the head of the Shanghai branch of the company. Hitchings and Moto become friends (Moto notices the letter), and Moto helps Hitchings cure a hangover. Hitchings complains to Moto that he has not met any beautiful women on board. After a stop in Honolulu, a beautiful woman named Gloria Danton boards the ship, and she and Hitchings fall in love. But Gloria is a spy for Nicolas Marloff, who runs a smuggling operation out of Shanghai. She periodically sends him notes and leaves without saying goodbye to Hitchings. Moto finds a steward looking for Hitchings’ letter and confronts him, knowing he was the person who killed the man in the wicker basket, as he wears the tattoo. Moto throws the man overboard and takes the letter.
A caravan settles for the night in the Gobi Desert. A man sneaks into a tent to steal a scroll, but adventurer and soldier of fortune Kentaro Moto (Peter Lorre) is only pretending to be asleep and kills him. When the caravan reaches Beijing, Moto is searched by the police. The scroll is found, but Moto grabs it and escapes.
In San Francisco, policeman Lieutenant Riggs (Harold Huber) takes Mr. Moto, a detective and Lee Chan (Keye Luke), a student, to a prizefight between Bill Steele (Dick Baldwin) and Frank Stanton (Russ Clark), where the winner will take on the champion, Biff Moran (Ward Bond). However, the fight is fixed and gangster Nick Crowder (Douglas Fowley) bets big money that Stanton won't make it to the fifth round. He goes down in the fourth and dies shortly afterward.
Over the ruins of Angkor Wat in Cambodia flies the airplane of aviatrix Victoria Mason, supposedly circumnavigating the world. Her actual destination is the kingdom of Tong Moi in French Indochina. Already in Tong Moi, Mr. Moto is posing as an archeologist, and newsreel cameramen Marty Weston and Chick Davis are traveling up the river. Once she is overhead, Mason lights a flare and bails out of her smoking plane which crashes nearby. Rescued by Moto, Mason goes into the village with Bokor the head priest of Shiva. As Weston and Davis arrive at the crash site, Moto has discovered the flare that “caused” the accident.
The film opens with a daring escape from the French penal colony on Devil’s Island. Mr. Moto, pretending to be Ito Matsuka, a Japanese murderer, is in the company of Paul Brissac, who belongs to a group of assassins. Brissac changes his name to Romero when they arrive in London and Moto stays on as his houseboy.
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.
