Charlie Chan takes his wife and twelve children on an outing to a circus after receiving a free pass from one of the owners, Joe Kinney. Kinney wants Chan to find out who is sending him anonymous threatening letters. Nearly all of the circus workers are suspects, since Kinney is very unpopular. However, when Chan goes to meet him during the night's performance, he finds the man dead, seemingly killed by a rampaging gorilla who somehow escaped from his cage.