The Chinese detective has three days to find the murderer in order to save an innocent man from being hanged.
Newspapers announce that Paul Gray (Douglas Walton) will hang for murder. His sister Pamela (Drue Leyton) pleads he is innocent but is rejected by the home secretary. She pleads with Charlie Chan (Warner Oland) to help her, and her fiancé Neil Howard (Ray Milland), who defended him in court, explains the case was based on circumstantial evidence, though he believes him guilty. This upsets Pamela so much she throws down the engagement ring.
Pamela goes to the home of Geoffrey Richmond (Alan Mowbray), where guests have gathered for a fox hunt. Chan comes to see Pamela but is turned away by the butler. After visiting Gray he climbs in her window. Chan collects the witnesses in the study. Several say they heard Gray quarreling with the victim, Captain Hamilton. Chan takes the witnesses to the stables in order to reconstruct the crime, a French technique. A horse gets upset, and the stableman Lake is afraid, saying he was not there then. Later the police detective Thacker (E. E. Clive) says Lake committed suicide, but Chan figures out it was murder. The poor Lake had money, which the murderer gave him to keep quiet. Major Jardine admits he was in the Air Force. Pamela visits her brother in jail, and Neil tries to explain to her. In the study someone shoots a dart at Chan. Chan tells Richmond, and Thacker finds the dart gun, while the police catch Neil on the grounds. Chan keeps Thacker from detaining Neil, saying, "If you want wild bird to sing, do not put him in cage." While Chan is questioning Air Force officers about Hamilton and his invention to silence war planes, Richmond's fiancé Mary (Mona Barrie) writes a note to Chan.
Neil tells Pamela he loves her. Chan, after reading the note, asks Neil to take him to the fox hunt in order to protect Mary. She is found unconscious, and Chan learns that the horse was blinded. Chan announces that Hamilton was murdered for his invention and that the murderer is in the room. Hamilton's plans may have the murderer's fingerprints. Gray is to be executed in four hours at 9 a.m. Chan does not find fingerprints on the plans; so he asks Richmond to wait with him for the murderer, handing him his gun. As Chan goes to the plans, Richmond shoots him. Police rush in, and he shoots again. Chan says, "Please excuse blank cartridges," and the police arrest Richmond. Gray is saved from the execution and thanks Chan, who presents the engagement ring to Pamela for Neil.
The polite detective uses Oriental wisdom to help him solve a difficult case quickly after he discovers the real motive.