Honolulu police detective Charlie Chan is awaiting the arrival of his first grandchild, and after he, his wife and their son-in-law, Wing Foo, rush to the hospital, a call comes in for him to investigate a murder on the freighter Susan B. Jennings . Chan's "number two" son James, who wants to become his father's assistant, is persuaded by his little brother Tommy to answer the call himself, and prove to their father that he is a good investigator. Tommy tags along as Jimmy goes to the freighter which has just arrived from Shanghai. There Captain Johnson assumes that Jimmy is Chan and explains that the murdered man's identity is a mystery, and that secretary Judy Hayes was the only eyewitness to the fatal shooting. Jimmy decides to question the rest of the freighter's passengers, who include animal keeper Al Hogan, Mrs. Carol Wayne, psychiatrist Dr. Cardigan, criminal Johnny McCoy, and policeman Joe Arnold, who is taking McCoy back to Shanghai. Judy reveals that her lawyer employer in Shanghai told her to deliver a package containing $300,000 to a man who would meet her in Honolulu. The man identified himself by a pre-arranged signal, but he was shot by an unknown assailant before she gave him the money. First mate George Randolph, who has fallen in love with Judy, takes Jimmy to question the surly crew members, and Jimmy is saved from their ire by Chan, who arrives after having found out about the case from Inspector Rawlins, his boss. Chan questions Carol, who states that she was on the freighter to rest while her suit for divorce was being heard, and that she recently became a widow anyway. Chan finds a wrapper for part of the missing money in the doctor's compartment, and becomes more suspicious of Judy when she slips off the ship to call her employer about the stolen funds. Carol reveals that Randolph gave Judy a gun with which to protect herself, and upon examination of it, Chan finds that it is the same caliber as the murder weapon. Judy then protests to Randolph that she is being framed when he questions her about the missing money that he found hidden in her cabin. Chan and Jimmy find Carol after she has been strangled with a scarf, and Chan reveals that Arnold is actually Mike Harrigan, who, while in league with McCoy, murdered the real Arnold after escaping from prison. While Chan and Cardigan rig a trap for the killer, Chan reveals to the passengers that Carol was really Mrs. Elsie Hillman, the dead man's wife, and Judy states that although she did not know about Carol, she was delivering the money to the man so that he did not have to declare it in a divorce settlement. Chan's trap works when the killer attempts to grab the murder weapon and triggers a camera, and after Cardigan develops the photograph, Johnson is revealed as the murderer. Chan explains that Johnson was after the money, and later killed Carol when she became suspicious. After the case is wrapped up, Chan receives a call from Wing Foo and happily listens as his grandson cries into the phone.