Brothers Amos and Dan Winterslip discuss Dan’s latest amour, a shady lady named Arlene Compton of whom the moralistic Amos does not approve. Dan, the family ingrate, throws his brother out of his beachside Honolulu house. His sister Minerva arrives with the news that another family member, cousin John Quincy Winterslip, is being sent to Honolulu to bring her back to Boston as, in the conservative family’s opinion, she is having too much fun. Dan sobers when he sees a newspaper item about the arrival to the islands of T. M. Brade.
During a stopover in San Francisco, John Quincy’s Uncle Roger takes him to a house and gives him a strongbox marked “TMB”. Roger instructs him to throw it into the Pacific the next day when his boat is out at sea, as per Dan Winterslip’s orders. While the pair is in the house, a mysterious figure jumps from the shadows and tries unsuccessfully to make off with the box.
Aboard the ship for Honolulu, John Quincy is about to toss the strongbox over the rail, but he is suddenly overcome by an unidentified person who quickly runs off with the box. On deck, Harry Jennison, Dan Winterslip’s lawyer, and Barbara, Dan’s Daughter, who are returning to Honolulu, decide to get married. An excited Barbara sends a radiogram to her father.
Meanwhile, in Honolulu, Dan goes to see Arlene to ask that she return the emerald brooch that he had given to her as a gift. She lies and says that the brooch is being repaired at a jeweler’s and promises to bring it to him later.
That evening, the captain of the passenger ship announces that, since the ship has arrived too late to dock, that it must remain at anchor offshore until the next morning. At Dan’s home, Minerva finds a prowler hiding in the shadows. Out of fear, she does not let on to the fact that she is aware of the intruder’s presence, although she notices that the person is wearing a distinctive watch that glows in the dark. After the prowler has left, Minerva discovers Dan dead in his den.
The police are called, and, at the police station, Captain Hallett receives the report of the murder. Hallett immediately calls Charlie Chan who is awakened along with his ample household. Soon, at the crime scene, the doctor reports that Dan was stabbed in the heart and that his arm was broken. Chan questions Minerva, who relates the only identifying characteristic of the intruder that she could make out: the glow-in-the-dark wristwatch, mentioning how she noticed that the numeral two was blurred. She also tells Chan that she remembers a James Eagan of the Reef and Palm Hotel calling repeatedly the day before the murder.
The next morning, at the hotel, Chan and Hewett, who are joined by John Quincy at his own request, are questioning a reticent Eagan who is very insistent on leaving. Chan reminds him that he had had an engagement with Dan that he had cancelled. Eagan admits that the two men had met at around 11:00 in Dan’s garden. Eagan, who had not spoken to Dan in twenty-three years, refuses to reveal to Chan the nature of their meeting.
Carlotta, Eagan’s daughter, walks in on the group, having just arrived in Honolulu aboard another ship. She is upset because her father was supposed to have met her at the dock, but she is grief-stricken to find that her father is now a suspect in the murder of Dan Winterslip. Carlotta rebukes John, who has openly stated his feelings of Eagan’s apparent implication in his uncle’s murder.
With her father in police custody, Carlotta must now run the hotel. As she watches the desk, Brade, a guest, informs her that he will be away for a few days. Koahla, Dan’s houseboy, enters with a package for Brade, but leaves when he is informed of the latter’s absence. Later, noticing Koahla hiding on the hotel grounds, John attacks him and retreives the package. He is stunned to find that it contains the strongbox that was stolen from him aboard ship. Soon, a shot is fired at John, narrowly missing him. Chan arrives, and John gives the box to the detective, who opens it only to find that it is empty.
Later, Amos Winterslip is found dead, and Chan captures a beachcomber who wears a wristwatch matching that described by Minerva. Shortly thereafter, Chan assembles all of the possible suspects at Dan’s house for a final meeting at which he plans to reveal the identity of the murderer. When Chan asks Brade about the box, Brade states that Dan Winterslip had stolen jewels from his father thirty-five years earlier when Dan was a mate on his father’s ship, and that he had scrimped and saved for all those years for the chance to reclaim his due. When the emerald brooch, which was found at the crime scene, is produced, both Arlene and Brade claim it. Brade describes matching jewelry that went with the brooch, which Chan soon discovers hidden in a secret compartment in the strongbox.
Chan then produces a check for $5,000 that Dan had made out for Eagan. When Egan still refuses to explain his relationship with Dan, Chan tells Hallett to arrest him. Carlotta cries out at the injustice and admits that she had read her father’s diary, which had told of an incident, thirty-five years earlier, in which Eagan, then a young bank teller in Australia, had changed Dan Winterslip’s South American gold into English pounds. Eagan, being the only man alive who could identify Dan as the thief of Brade’s gold, had tried to blackmail him. Koahla then admits that, knowing of Dan’s fear of Brade, had arranged for the theft of the strongbox in San Francisco in order to blackmail Dan.
Chan then has his eldest son, Oswald, bring in Berkeley, another passenger on the boat, and claims that he is the murderer, as water and seaweed which were found at the murder scene prove that someone swam from the ship on the evening in question. Berkeley suddenly tries to make a run for it, but he is caught by Jennison, who breaks his arm in the same fashion that the late Dan Winterslip’s arm had been fractured. Chan dramatically accuses Jennison of being the real murderer, stating that Berkeley was bravely playing an assigned role.