Botany student Harold Bledsoe is bound for San Francisco to help investigate a crime wave in the city's "
Chinatown" district. Since Harold is the son of San Francisco's former police captain, municipal authorities hope he will be as skilled as his father in solving crimes. Also traveling to the city, but by car, are a young woman named Billie Lee and her little brother Buddy, who needs his lame leg treated in San Francisco by "the famous Chinese physician" Dr. Chang Gow. During a brief train stop in
Colorado, Harold has his photograph taken at a
vending machine. He is surprised to see a
double-exposed print of himself and Billie Lee. He finally meets Billie when her car breaks down and she removes the
carburetor. A motorist gives them some gasoline, but after he leaves, Billie realizes that she left the carburetor on the other car's running board. Now Harold and the Lees must spend the night along the roadside. Next morning, the trio harnesses a cow to pull their car to a gas station. Harold then catches another westbound train. Once in San Francisco and at police headquarters, Harold is fascinated by
fingerprinting, and takes prints of everyone at the building, including respected citizen John Thorne, who is pressuring the police to crack down on crime. The desk sergeant gets rid of Harold by assigning him to find the "Dragon," the mysterious master of the city's Chinese underworld. To "aid" Harold in his search, the sergeant gives him Thorne's fingerprint, but he lies and tells him it is the Dragon's print. Harold goes to Chinatown and sees Billie, who gives him the address where she and Buddy are staying. Harold gets a potted flower for Billie. Dr. Gow, examining Buddy's leg, accidentally breaks the flowerpot, revealing a package of
opium. The doctor goes to the flower shop, where he is kidnapped. Harold leaves to rescue the doctor. In Chinatown, Harold and Clancy, a street cop he had met earlier, go to the flower shop. Aware of their presence, employees there set up a series of spooky effects to frighten them from the premises. Harold soon encounters the masked Dragon and some of his men preparing to execute Dr. Gow. Harold struggles with the Dragon, who escapes with his hostage before the police burst in and arrest everyone else. Harold recognizes Thorne's fingerprint and denounces him, but the police think Thorne is above suspicion. They release Thorne and detain Harold. Harold gets away and follows Thorne to his home, where he eventually extracts a confession from him. The police arrive and remain skeptical until Harold finds Dr. Gow bound and gagged in a closet in Thorne's study. Thorne is arrested, and Billie accepts Harold's clumsy proposal of marriage. ==Cast of the sound version==