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Charlie Chan takes his wife and twelve children to the circus owned by kindly John Gaines and his unscrupulous partner, Joe Kinney. Kinney tells Chan that he has been receiving death threats, after which Kinney and Gaines quarrel over money. Kinney then tries to discipline Caesar, the circus' ape, but the handler, Hal Blake, warns Kinney that he treats Caesar too roughly. Hal seeks solace from his girl friend, Louise Norman, whose sister, aerialist Marie, is Kinney's fiancée. As the show goes on, Chan leaves his family to meet Kinney and discuss his problems. When he arrives at Kinney's wagon, however, Chan, Gaines, and dancing midgets Tim and Tiny discover that Kinney has been murdered. Because the wagon was locked from the inside, and because of the hairs found on the window sill, the others suspect that Caesar climbed through the window and killed Kinney. Chan, however, withholds judgement and turns the case over to Lieutenant Macy, then leaves with his family to continue their vacation. Later that night, however, Tiny appears at their hotel and pleads with Chan to continue the investigation to clear Tim and Gaines, who are being held for questioning. Chan convinces Macy to release Tim and Gaines and to allow the circus to travel on, in hopes that the killer will reveal himself as time passes. Chan, his son Lee and Macy travel with the circus and, despite an attempt on Chan's life that night, reach their destination the next day. They discover that the business wagon's safe has been broken into, and in the safe they find Kinney's insurance policy, naming Marie as the beneficiary, and a marriage certificate stating that Kinney and wardrobe mistress Nellie Farrell were married in Juarez, Mexico on 30 May. The trio later confronts Nellie and her brother Dan, and Nellie asserts that, as Kinney's widow, she is entitled to his half of the circus. Marie retorts that Nellie's claim is false, as Kinney could not have been in Juarez on that date, but before she can offer proof, she has to perform her act. While Marie is in the air, someone shoots her rigging and she falls to the ground. She is seriously injured and the doctor Chan summons states that she must be operated on immediately. While the doctor is tending to Marie, Chan looks through her scrapbook and discovers that on 30 May, Kinney was being held as a witness to a murder in El Paso. While Lee telephones the El Paso police for more information, Caesar is released from his cage. Caesar attempts to kill Marie during the operation but is caught, after which Chan reveals that it is not the true Caeser who was apprehended, but snake handler Tom Holt, disguised as the ape. The police confirm that Holt was the killer in El Paso, and Chan deduces that Kinney covered up for Holt but was later murdered by Holt when they quarreled over money. Holt then attempted to murder Marie, who is safely recovering at a hospital, because she could reveal the El Paso incident. Holt is arrested, along with Nellie and Dan, who forged the marriage certificate after Kinney's death. Gaines promises Chan lifetime passes to the circus as a reward for his help, and Lee romances Su Toy, a pretty Asian contortionist whom he had been pursuing throughout the investigation.