On a train through Europe, Iris Henderson meets the kindly old woman Miss Froy, and they meet several other passengers over the course of their conversation. Iris later wakes up from a nap to discover that Miss Froy is nowhere to be found, and none of the people they met seem to have any recollection of her. A psychiatrist on the train suggests that Miss Froy never existed: Iris was bumped on the head before boarding, and the conversation may have only taken place in her head. However, Iris is certain that something more sinister is going on, and teams up with another acquaintance, the musician Gilbert, to find her before the train reaches its destination.
A boxer dies in the prizefight ring, and detective Mr. Moto is called in when it turns out to be murder. What Moto uncovers, with the help of Charlie Chan's son Lee, is a gangland gambling racket.
Mr. Moto poses as both an innocuous archaeologist and aged Buddhist mystic in French-ruled Cambodia in order to foil two anti-government revolutionary plots, one by fanatical holy man Bokor, who wants to overthrow the local leader, the self-indulgent Rajah Ali, and Ali himself, who wants to lead an insurgency against French colonial rule. Moto is aided by Vicki Mason, a daring Amelia Earhart-type aviatrix, who doubles as an international agent, and manages to rescue two bumbling newsreel photographers, Chick Davis and Marty Weston, the latter of whom seems to be attracted to the stunningly beautiful Miss Mason.
Mr. Moto has himself imprisoned on Devil's Island so he can help his cellmate (Ames) escape and thereby get the goods on a gang of international killers.
Mr Moto encounters mysterious goings-on on a ship bound for Shanghai. He recognises his steward as the murderer of a man in San Francisco, and catches him trying to steal an important letter from the stateroom of another passenger, Robert Hitchings. Hitchings, son of the owner of the shipping line, falls in love with Gloria, who refuses to tell him anything about her life and disappears when they arrive in Shanghai. In Shanghai, Mr Moto uncovers the secret which links the murder in San Francisco, the mysterious letter, and Gloria.
Mr. Moto finds himself in Peiping seeking ancient scrolls that, placed together, will form a map to the location of Genghis Khan's long-hidden treasure. There are a total of seven scrolls and Moto himself has recently acquired one of these during a foray into Mongolia. The remaining six scrolls are in the possession of Madame Chung and her son, Prince Chung. They very much want to protect the dignity and honor of their ancestors and with Moto's help, try to keep thieves, led by Colonel Tchernov, from getting their hands on the map.
So, famed and honored detective Charlie Chan (Warner Oland, surely the best of all the Chans)is given fourteen free tickets to the circus - for him, his wife and their twelve children. And it appears they visis tthe circus at just the right time to get drawn into investigating the murder of one of the circus owners. It seems more than Charlie's usual perfect detective work is needed in this unusual case. The solution is quite incredible!
While steaming from Honolulu to Los Angeles the owner of a prize racehorse headed for the Santa Anita Handicap is killed, apparently kicked to death by his stallion. Not so, deduces Charlie. Leter he exposes efforts to fix a race at the famous track.
