In this intriguing adventure, Peter Lorre plays the resourceful and sharp Mr. Moto, a master of disguise who finds himself embroiled in a web of mystery and danger. The story begins with Moto blending into the bustling streets, disguised as a street salesman, offering various goods to passersby. During his walk, he notices a man leaving a shop with a striking tattoo of the British flag on his arm, which will later prove to be a crucial detail.
Inside the shop, Moto aims to purchase a rare diamond but witnesses something far more sinister— a body packed into a wicker basket. Using his impressive expertise in judo, he quickly subdues the shopkeeper, revealing a hidden criminal activity. This early encounter sets the tone for a plot filled with deception, espionage, and danger.
Mr. Moto then secures a stateroom on a passenger liner headed for Shanghai, where he encounters Virginia Field as Gloria Danton, also known as Tanya Boriv, a woman with a mysterious background. Among the passengers is Thomas Beck as Robert 'Bob' Hitchings, Jr., the son of a shipping company owner. Before departure, Hitchings receives a secret letter from his father, Murray Kinnell as Joseph B. Wilkie, intended for the head of the Shanghai branch, which Hitchings is entrusted to deliver. During the voyage, Moto and Hitchings form a friendship — Moto even helps Hitchings recover from a hangover, and Hitchings confides in Moto about his difficulties in meeting attractive women.
As the ship makes a stop in Honolulu, a captivating woman— Gloria—boards the vessel. Beneath her glamorous exterior lies a spy working for Nicolas Marloff, played by Sig Ruman, who is involved in a clandestine smuggling operation out of Shanghai. Gloria and Hitchings quickly develop a romantic connection, but their budding relationship is complicated by her secret identity and her covert communications with Marloff, which she leaves behind without farewell.
The plot thickens when Moto discovers that the assigned letter is a blank sheet of paper, raising suspicions about its authenticity. He also uncovers that the tattooed man he saw earlier was connected to the hidden body, leading to a confrontation where Moto throws the suspect overboard and retrieves the letter, revealing his keen investigative skills.
Arriving in Shanghai, Hitchings meets with Murray Kinnell once again but soon realizes the contents of the letter are missing or tampered with. With his suspicions growing, Hitchings is determined to find Gloria. He receives an anonymous tip directing him to the International Club, where he discovers her performing on stage. When Hitchings approaches her in her dressing room, the club owner— Marloff—covertly notices their interaction and promptly locks them both up, fearing he’s uncovered too much.
J. Carrol Naish as Adram, Moto confronts Marloff under the guise of a fellow smuggler, tricking him into revealing Gloria’s hiding spot and leading to their rescue. Meanwhile, Lola Liu, played by Lotus Long, tries to contact police but is shot while doing so, leaving a vital message about their location.
The climax unfolds as Moto summons the police, and Moto’s quick thinking results in Marloff’s arrest after Moto captures him attempting to escape. Wilkie, who has been involved in the smuggling scheme, is exposed when he reveals that he replaced the secret letter with a blank page and shot Lola Liu. In a tense showdown, Wilkie is ultimately caught, and Marloff’s true role in the crime ring is exposed.
The story concludes with justice prevailing— Wilkie is arrested, and the criminal operation dismantled. Throughout the film, Mr. Moto’s resourcefulness and moral integrity shine through as he navigates dangerous waters filled with deception, espionage, and treachery, ensuring that righteousness and justice are restored in the end.
