Chan's son helps the detective solve a complicated double murder on board a ship.
At family dinner Jimmy Chan (Victor Sen Yung) tells his father Charlie Chan (Sidney Toler) that he wants to be a detective. Wing Foo tells Charlie that his daughter is going to have a baby soon, and they go to the hospital. Jimmy responds to a call about a homicide on a ship with Willie Chan (Layne Tom Jr.). Captain Johnson (Robert Barrat) greets Jimmy as Charlie Chan, who learns of the case from Inspector Rawlins (Paul Harvey). While Dr. Cardigan (George Zucco) and Carol Wayne (Claire Dodd) listen, Jimmy questions Judy Hayes (Phyllis Brooks), who gave $300,000 to the victim. Jimmy finds Willie in a closet, and Willie says Dr. Cardigan did it. Jimmy tries to question the rough crew and is exposed by police as a fraud; but Charlie prevents his ejection. Charlie shows that Dr. Cardigan is not deaf. Jimmy gives the victim's ring to Charlie, who reads the initials. Charlie questions Judy, Carol, and George Randolph (Dusty King). Arnold (Richard Lane) says that he is extraditing Johnnie McCoy (Marc Lawrence). Charlie orders all to stay on board.
Carol tells Randolph that she saw him give the gun to Judy. Arnold offers to help Charlie, but Judy slaps him. Al Hogan (Eddie Collins) tries to stop his lion from attacking a monkey. Charlie learns that Dr. Cardigan is keeping a brain alive, and Hogan asks the doctor to examine his head. Charlie sees McCoy come back with the door key, and Arnold hits him. Arnold does not want Charlie to know that he is Mike Hannigan. Charlie calls Rawlins that Judy escaped; but she is brought back, and Carol makes her give up the gun Randolph gave her. Randolph tells Judy that he found $10,000 in her cabin, but she says it was planted. Carol finds Dr. Cardigan in Judy's cabin while Charlie and Jimmy are searching Carol's cabin. Charlie suspects that Carol was married to the victim. Charlie and Jimmy find Carol dead. Captain Johnson reacts to a fire on the ship; but Charlie puts it out and discovers it was set by Jimmy to flush out the murderer. Jimmy saves Charlie from being shot. McCoy gets the money from a lifeboat but is caught by Randolph. Charlie arrests McCoy and Hannigan. Hogan brings in the gun that was fired. Charlie gets Dr. Cardigan to pick up the gun and asks about a money band found in his cabin. Charlie asks Captain Johnson why he let Carol travel under a false name. Judy admits the money was for Carol. When the lights go out, someone takes the gun; but Charlie got a photo. Dr. Cardigan develops the photo of Captain Johnson, and Charlie explains how and why the captain committed murder. In the final scene Charlie learns his grandson is born.
Wise Charlie helps guide his energetic sons and patiently solves the mystery, proving his proverb that caution is a good life insurance policy.