Charlie Chan at the Olympics 1937

Charlie Chan is proud of his oldest son, swimmer Lee Chan, for being chosen to represent the United States at the 1936 Summer Olympics in Berlin. Meanwhile, over Chan’s Honolulu skyline, the “Hopkins plane” is demonstrating an improvement in remote radio control for the US military. The aircraft is hijacked by a concealed stowaway, the device stolen, and the test pilot murdered. When Chan and his astute second son, Charlie Chan Jr., identify the stowaway, he finds only the man’s body.

On the passenger list of the only aircraft to leave Honolulu for the mainland after the incident are Richard Masters and Yvonne Roland. Masters was the intended test pilot, but injured his shoulder and had to be replaced. He left on the Pan Am Clipper to travel to the Olympics himself as a competitor. Perusing the list, Hopkins recognizes the name of arms dealer Arthur Hughes. Several of the suspects head for Germany on SS Manhattan (1931) with the US Olympic team. Chan, Hughes, and the inventor Cartwright arrive before them by taking the Zeppelin Hindenburg.

Aboard Manhattan, Lee Chan suspects Yvonne Roland of being an “adventuress.” He spots her putting a piece of paper in her book, and Hughes taking it. He confides in fellow Olympian Betty Adams, Masters’ girlfriend.

When the ship docks, German Inspector Strasser, Chan, and Hopkins go to question Roland, only to find her gone and her cabin rifled. Chan is reunited with his son when the latter sneaks into the cabin through a porthole to play detective.

On the train trip to Berlin, Chan learns that Lee saw Roland borrow a camera from Adams. Hughes overhears the pair speculate that the invention was smuggled off the ship in it. Hughes advises Chan to stop investigating, then spots a gun barrel in a passing car and pushes the pair to safety. A shot strikes their compartment window. Later, Hughes arranges for Adams’ camera to be stolen, but finds nothing inside.

A hotel maid tries to steal the invention, hidden in Adams’ candy box, but is foiled by Chan and Strasser. When she runs to the window to warn her accomplices, Charlie substitutes a book for the device without being noticed. Hopkins insists on keeping the candy box in his hotel suite, where he is visited by Hughes. Chan and a policeman break in when they hear gunfire. They find Cartwright on the floor. He claims Hughes accused Hopkins of taking the device to double-cross the stockholders of his company, then after Hughes left, Hopkins took the invention at gunpoint and slugged him.

Roland takes the candy box to her employer, diplomat the Honorable Charles Zaraka. When they discover the substitution, Roland surmises that Chan has the device. Zaraka sends Chan a ticket to the Olympic opening ceremony. While Chan verbally spars with him and Roland, Lee is kidnapped.

Chan allows himself to be taken to Zaraka to exchange the device for his son. Hopkins is brought in to verify it is the invention; he does (even though it is an imitation with a radio beacon inside). Satisfied, Zaraka orders his men to dispose of the Chans when Hughes and his men burst in. Hughes threatens to kill Lee if Chan does not produce the real equipment, but the police arrive just in time to rescue the detective. Hopkins is found unconscious from a gunshot in another room. Cartwright accuses Hughes of the shooting, but Chan proves that Cartwright was responsible for that and also the murder of his accomplice back in Honolulu. Afterward, Lee wins the 100 meter swim.