Detective Charlie Chan's daughter prepares to give birth to his first grandchild. While Chan waits at the hospital, his "number two" son Jimmy intercepts a message intended for Charlie about a murder aboard the freighter
Susan B. Jennings. The freighter is on its way from Shanghai to Honolulu under the command of Captain Johnson (
Robert Barrat). Jimmy, wanting to prove his investigative skills, boards the ship pretending to be his father, with his younger brother Tommy (
Layne Tom Jr.) in tow. The ruse doesn't last long and soon the real Chan arrives on board, interrogating a motley assortment of crooks, heiresses, and crew members as he works to solve a crime whose only witness is secretary Judy Hayes (
Phyllis Brooks). The suspects include the eccentric scholar Dr. Cardigan (
George Zucco), who claims to have kept a human brain alive in
formaldehyde for six months and is always looking for new subjects; Mrs. Carol Wayne (
Claire Dodd), embroiled in a bitter divorce settlement involving $300,000 in stolen cash; and criminal Johnny McCoy (
Marc Lawrence), who is being extradited to the United States in the custody of detective Joe Arnold (
Richard Lane). Captain Johnson is eager to unload his cargo, which is mostly a shipment of lions destined for the San Francisco Zoo. Deckhand Al Hogan is fond of walking his pet lion Oscar on the main deck, to nobody's pleasure. Jimmy starts a fire on the ship, hoping to panic the killer in grabbing the hidden money and making a run for it. Chan's anger is cooled when Jimmy shoves him out of the way of an assassin's bullet. Chan recovers the gun and announces that the murderer will be revealed by the fingerprints on the gun. The lights go out for a few moments, and now the gun is gone. But while stealing the gun in darkness, the guilty party triggered an infra-red camera that identifies the culprit. The story ends with the remaining passengers gathered around the ship-to-shore phone, listening to the coos and cries of Charlie Chan's first grandchild. ==Cast==