The film is also known as Charlie Chan in the Scarlet Clue (American informal title) and Charlie Chan: The Scarlet Clue in Australia. The film is in the public domain due to the omission of a valid copyright notice on original prints.
Plot
Charlie Chan is working for the United States Government tracking down the theft of radar secrets. When the man they are tailing is murdered, the only clue is a footprint in blood. Chan. His "Number Three Son" Tommy and their chauffeur Birmingham Brown's investigation leads to a radio station. Birmingham runs into his old friend Ben Carter at the station with other suspects including the radio station staff, stars and cleaning woman. The closer Chan gets to solving the mystery, the more mysterious murders happen.
The science lab where the radar secrets are developed shares the same skyscraper floor with a radio soap opera studio, the program
apparently under the thumb of its tyrannical sponsor, Mrs. Marsh. The lab also has a weather chamber that can create below zero blizzards or extreme heat. Tommy Chan and Birmingham spend a great deal of time comically trapped in it.
It is quickly established that the station manager, Ralph Brett, is part of the spy ring. But he only communicates with the Master Spy by a clever series of telephone relays. The leader has invented a clever poison that causes death in conjunction with lighting a cigarette. Blackmailing actress Gloria Bayne and genial ham actor Willie Rand meet death by this device.
When the Leader judges Brett to be a liability, he is lured to a freight elevator with a trap-door floor that drops him seven stories to his death.
Unnerved by the murders, another member of the ring offers to lure the Leader into the open with a false distress message.
During an in-and-out corridor chase on the 7th floor, the Leader is revealed to be Mrs. Marsh...who dies when she panics and
accidentally steps into her own elevator trap.
