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Disguised as Ito Matsuka, Japanese murderer Kentaro Moto, an agent of the International Police, leads Paul Brissac, a Frenchman, in an escape from Devil's Island.  In London, Moto becomes a houseboy for Brissac, a member of a group of hired killers of diplomats and economic leaders, which the newspapers have dubbed "The League of Assassins."  Moto is trying to learn the identity of the group's leader and the reason that they are in London.

Meanwhile, Prague steel king Anton Darvak, a pacifist, has refused to sell to armament manufacturers a formula for a new type of steel.  After he receives a threatening telephone call at his London hotel, Moto learns of the call from one of his agents, Lotus Liu.  On his way to visit Darvak, Moto witnesses a lorry back up and kill a man in a street market.  Darvak at first refuses to tell Moto about the threat against his life, despite the urging of his secretary, Ann Richman, and his business associate, David Scott-Frensham.  However, when he learns that the man run down by the lorry, Lord Gilford, was killed as a warning to him, Darvak confides to Moto that he was told he would be killed the next afternoon at three if he does not part with the formula.

Moto visits the Limehouse tavern where Lotus lives.  After he hears the accordion player play the same tune that an organ grinder played before Lord Gilford was killed, a brawl begins.  Moto takes Lotus to her room to pack, and they find the body of a member of the league, who was killed because he confided in Lotus.  At Brissac's residence, Ernst Litmar tells Brissac that Moto was seen in the bar speaking with Lotus.  They send Moto to a certain store for fruit, and when Moto gets there, he hears a street violinist play the tune he heard before the two previous murders.  A number of toughs crowd around him, but he escapes in a cab.

The next day, Moto goes to Coventry Galleries, knowing that Darvak plans to be there that afternoon at three to see an exhibition.  He overhears Litmar explain to Brissac the plan to kill Darvak: the leader of the group will position Darvak in front of a particular painting and say his name in a loud voice; the orchestra will then play the same tune used previously to identify men to be murdered; Brissac, in the gallery loft, will cut the chandelier, and its fall will kill Darvak.  After Ann tells a surprised, but very pleased Darvak that she loves him, Moto visits.  David also arrives and says that he has brought two men from Scotland Yard to protect Darvak.  When Moto is stopped by the two plainclothesmen in the hall, he sees that one of them has a gun and knocks them over a banister.

As three o'clock nears, David arrives alone at the gallery, and as he stands in front of the painting, an eccentric German artist complains about the art exhibited.  When Darvak arrives with the plainclothesmen, the crank artist loudly calls David "Darvak," and the orchestra plays the tune.  Brissac cuts the chandelier, and it kills David.  The crank artist then removes his disguise and reveals himself to be Moto.  He explains he knew that the leader must be David because the plainclothesmen had guns, while Scotland Yard officers do not carry any.  As the gang is apprehended, Brissac shoots from the loft, but Moto fights and subdues him.

Notes
The film is based on the character "Mr. Moto" created by John P. Marquand.

This film was also reviewed under the title Mysterious Mr. Moto of Devil's Island.  According to information in the Twentieth Century-Fox Records of the Legal Department at the UCLA Theater Arts Library, twenty feet of stock footage entitled "Chase Through Devil's Island" was purchased from Columbia Pictures for use in this film.

According to publicity, director Norman Foster disguised himself as a hoodlum for the brawl scene in the Limehouse tavern.  According to an HR news item, Michael Whalen was originally cast for the role of "Anton Darvak," but he was needed for more shooting in Racing Blood , the working title of Speed to Burn.

Music includes:  "It's the Syme, the Whole World Over," music and lyrics by John Paul Lock Barton and Bert Massee; "Black Black Sheep," music by Louis De Francesco, lyrics by Frank Tuttle.  The song "Black Black Sheep" was used earlier in Fox's 1934 film Springtime for Henry.

This film had a Los Angeles preview for the trade press on May 27, 1938, months before its national release.  HR commented in their review, "The progressive development of the Mr. Moto pictures has been one of Hollywood's most interesting evolutions of series films during the past year."