Chan and his son discover the murderer of a circus owner.
Charlie Chan (Warner Oland) attends the circus with his wife and twelve children. Joe Kinney (Paul Stanton) greets Chan and says he has threatening letters. John Gaines (Francis Ford) tells Kinney he may not be able to pay the notes he owes at the end of the season. Kinney whips the ape, and in a fight a key to the cage is lost. Hal Blake (John McGuire) asks Louise Norman (Shirley Deane) to leave the circus with him. Lou's sister Marie Norman (Maxine Reiner) wants to marry Kinney, while Nellie Farrell (Drew Leyton) complains to her brother Dan Farrell (Boothe Howard) about Kinney. Chan looks for Kinney, but he is found dead in a locked wagon. The ape is loose and chases Lee Chan (Keye Luke), who puts Su Toy (Shia Jung) in a cage to protect her. Charlie Chan finds ape hairs on the window. The ape jumps on him but is captured. Police lieutenant Macy (Wade Boteler) shows Lee how the ape could break a neck by the window. Charlie says the ape was released by the lost key. The midgets Tiny (Olive Brasno) and Tim (George Brasno) ask Charlie to help Gaines, because Macy closed the circus by arresting Gaines and others.
Charlie Chan suggests that Macy let the show go on in order to find the killer. On the train Charlie and Lee Chan stay in Kinney's room. A snake is dropped in, but Lee shoots it. Snake-tamer Tom Holt (J. Carrol Naish) says he is being framed. Someone also broke into the business wagon. Macy finds Kinney's insurance policy with Marie as the latest beneficiary and a marriage certificate to Nellie. Lee dresses as a woman to follow Nellie in town to a lawyer. By examining bronze filings, Charlie says someone opened the safe and an envelope. Nellie tells Gaines that she owns Kinney's share of the show because of their marriage. Marie denies the marriage, and she is later shot off her trapeze, putting her in a coma. Charlie shows that Kinney's signature was traced. He finds a clipping of an El Paso murder and has Lee call their police. While Marie is being operated on, Lee fights a man releasing the ape but is knocked out. The ape disrupts the operation and is shot. Charlie explains that the operation was a deception. Lee says that Tom Holt matches the description of the El Paso murderer. Charlie shows Holt dead in the ape suit and explains why he committed murder in El Paso and at the circus.
As usual Charlie Chan sprinkles wisdom and humor into his detective work. "Good tools shorten labor" and "Mind like parachute - only function when open." are two examples. His son Lee wants to be a detective, but half the time he is distracted by the charms of contortionist Su Toy. Thus a detective story is given style and charm.