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 Honolulu house, and 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 family's opinion, she's having too much fun. Dan sobers when he sees a newspaper item about the arrival to the islands of a T. M. Brade. During a stopover in San Francisco, John's uncle Roger gives him a strongbox marked "T. M. B." and instructs him to throw it into the Pacific the next day, per Dan's orders. As John is about to toss the box into the sea, he is tackled by an unidentified man who steals the box. On deck, Harry Jennison, Dan's lawyer, and Barbara, Dan's daughter, also traveling back to Honolulu, decide to marry and telegram Dan with the news. When Dan goes to see Arlene to ask that she return an emerald brooch he had given her as a gift, she lies and says that the brooch is at the jeweler's being repaired and promises to bring it to him later. That evening, the captain on the passenger ship announces a smallpox quarantine that requires the passengers to stay on board until morning. At Dan's home, Minerva finds a prowler and then discovers Dan dead in his den. Captain Hallett at the police station receives the report of the murder, and police detective Charlie Chan is woken up along with his household of several children. At the crime scene, the doctor reports that Dan has been stabbed in the heart and that his arm has been broken. Chan questions Minerva, who remembers only one identifying characteristic of the prowler: a glow-in-the-dark wristwatch with a blurred numeral two. She also tells Chan that she remembers a James Eagan of the Reef & Palm Hotel calling repeatedly the day before the murder. At the hotel, Eagan tries to leave, but Chan reminds him that he had an engagement with Dan, which he had canceled. Eagan admits that Dan insisted on seeing him the night of the murder and that the two met after 11:00 in Dan's garden. Eagan, who hadn't spoken to Dan in twenty-three years, refuses to reveal to Chan the nature of their business transaction. Carlotta, Eagan's daughter, whom John had met on the boat to Honolulu, rebukes John, who has accompanied Chan to the hotel, for questioning her father. Meanwhile, as Carlotta watches the desk of the hotel, Brade, a guest, informs her that he'll be going away for a few days. Koahla, Dan's houseboy, enters with a strongbox for Brade, but leaves when he discovers the latter's absence. Seeing Koahla hiding in some bushes on the hotel grounds, John attacks him and retrieves the box. John now gives the strongbox, which he had earlier tossed into the ocean, to Chan, but they find it empty. Chan finds Amos' dead body on the grounds of the hotel and captures a beachcomber who wears the glowing wristwatch. Later, Chan assembles all 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 says that Dan stole jewels from his father thirty-five years earlier when Dan was a mate on his father's ship, and that he has scrimped and saved for many 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. Chan then produces a check for $5,000 which Dan had made out to Eagan, and when Eagan still refuses to explain his relationship with Dan, Chan tells Hallett to arrest him. Carlotta cries out at the injustice and admits that she read her father's diary, which told of an incident thirty-five years earlier in which Eagan, a young bank teller, changed Dan's South American gold into Australian currency. Eagan, being the only man alive who could identify Dan as the thief of Brade's gold, tried to blackmail him. Koahla then admits that he knew of Dan's fear of Brade and stole the box in San Francisco in order to blackmail Dan. Chan then has his son bring in Berkeley, another passenger on the boat, and claims that he is the murderer, as water and seaweed found at the scene of the crime prove that someone swam from the ship on the evening in question. Berkeley tries to make a run for it, but he is caught by Jennison, who breaks his arm in the same fashion that Dan's arm was fractured. Chan then dramatically accuses the real murderer, Jennison, and admits that Berkeley was playing an assigned role. Chan also reveals that Dan's reply to Barbara and Jennison's wedding announcement never reached Barbara and contained a warning to Jennison that unless he broke the engagement, Barbara would be disinherited and the robbery exposed despite injury to both Jennison and himself. Tan lines on Jennison's wrist reveal that he is indeed the owner of the watch, which he lost in the surf, where the beachcomber picked it up following the murder. Jennison pulls an unloaded gun from Chan's pocket, and Chan has no difficulty subduing him. The caper solved, John and Carlotta embrace contentedly in front of the Reef & Surf Hotel, as Chan and family drive by in a new car, a gift from Minerva.