Charlie Chan of the Honolulu Police, and Inspector Fife of Scotland Yard tour the offices of the New York Police Department with Inspector Flannery, in order to study the methods used by that department.
While dining with Chan in a Chinese restaurant, Fife receives a telephone call informing him that Sir Lionel Grey, former chief of Scotland Yard, has dropped dead in the penthouse apartment office of Barry Kirk in Wall Street. Grey had been using Kirk’s residence as his base of operations while investigating a case. As Fife and Chan meet Flannery at the scene, Kirk informs the group that, before his death, Grey was about to solve a big murder case that had baffled Scotland Yard for years, and had invited several guests to a party. He had left the party to take a phone call in Kirk’s office where he died suddenly and mysteriously.
The doctor on the scene assures the group that Grey died of a heart attack, but Chan notices a dead cat in the room, stating, “Cat is like rich man’s heir – never dies out of sympathy.” The detective surmises that Grey’s death was not natural, and, whatever method was used to kill Grey also killed the cat. After Fife asks Chan if the simultaneous deaths may be mere coincidence, Chan replies, “I always suspect coincidence – same as nose always suspect ancient cheese.”
The assembled guests are all questioned, and each person seems to be hiding something. In the office, the police discover that the safe has been robbed – a possible motive for the crime – and learn that John R. Douglas, a chemical manufacturer, had made the last phone call to Grey. Even though Chan has plans to return to Honolulu on the birth of his newest baby (the Chans’ eleventh – a boy), he decides to stay on for the investigation.
At a cafe, Shirley Marlowe meets with John Douglas, and he tells her that Sir Lionel Grey had demanded to know the whereabouts of an Alan Raleigh, threatening to take Shirley, Raleigh’s former lover, back to England as an accessory to murder if John would not give the information. John then asks Shirley to marry him, stating that the two of them can go away together.
After an interview with one of the assembled guests, Chan seeks out and finds Shirley, a Follies performer at a theater where she is a well-known masked dancer. Shirley confesses to the detective that, years ago, she had fallen in love with Raleigh before she had discovered the crime that he had committed. When she had learned of the crime, Shirley fled, not wishing to incriminate a man whom she had once loved, and she has been pursued around the world by Scotland Yard ever since. Chan promises to keep Shirley’s secret, and goes out to find Li Gung, Raleigh’s servant, whom Shirley had mentioned during their conversation.
Locating Li Gung’s residence, with the help of information given to him by Kee Lin, owner of the Chinese restaurant in which Chan and Fife had dined, the detective gains no useful information, and has only succeeds in arousing the suspicions of Li Gung.
Back at Kirk’s office, it is revealed to Kenneth Dunwood, another guest on the night of the murder, and Kirk, that gas masks were found at Douglas’ chemical factory.
Back at the theater, Shirley writes a note to Chan, stating that she will tell the police everything. The note is given to her chauffeur to deliver to the detective. However, an unidentified man drops a small poison gas-filled bottle into the car, killing the driver.
At the police station, John is brought in. He denies his acquaintance with Shirley, until she tells him that she has confessed to everything. John then informs the group that he had seen a “Chinaman” enter the building with a basket on the day that he had met with Grey.
Chan then returns to the home of Li Gung where the latter attempts to kill him using an elaborately contrived trap involving a hidden gun. However, at the last moment, a black cat nudges the gun, which causes it to point instead at Li Gung, who falls victim to his own trap.
Later, at the check-in room at the Cosmpolitan Club, Chan discovers Sir Lionel’s briefcase, which he determines from the register, was checked in after the murder. Fife and Flannery watch with him who comes to retrieve it, and the trio is surprised to see that it is Barry Kirk who picks up the important item.
After Kirk has explained that he was picking up the briefcase for someone else, Chan, Fife, and Flannery secretly wait in Kirk’s office, along with Shirley, and Kirk is instructed to answer the door and pretend that he is alone. Soon, Kirk opens the door for Dunwood who thanks Kirk for getting his briefcase at the club. Kirk asks Dunwood when he had gotten his membership card to the club, and it is revealed that Dunwood had not been to the club that day as he had told Kirk earlier, but that he had really dropped off the briefcase just after Grey had been murdered.
After Shirley identifies Dunwood as Alan Raleigh, Dunwood grabs a gun that Chan had “clumsily” dropped on the floor while “sneezing.” After confessing to Grey’s murder as he holds everyone at gunpoint, Dunwood attempts to escape but Chan subdues him as Dunwood tries to shoot the detective with the gun that is actually unloaded. “Old habit,” Chan tells a relieved Inspector Fife, “wife never likes loaded gun, on account of children.”