Detective Chan, Sidney Toler, rushes to the hospital to be with his daughter as she prepares to give birth to his first grandchild. While Charlie Chan waits at the hospital, his “number two” son Jimmy intercepts a message intended for Charlie about a murder on board the freighter Susan B. Jennings.
The freighter is on its way from Shanghai to Honolulu under the leadership of Captain Johnson, Robert Barrat. Jimmy boards the Jennings pretending to be Charlie Chan, with his younger brother Tommy Chan, 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 as he works to solve a crime whose only witness is secretary Judy Hayes, Phyllis Brooks.
Captain Johnson is eager to unload his cargo, which is mostly a shipment of “big cats” destined for the San Francisco Zoo. Deckhand Al Hogan, Eddie Collins, is fond of walking his pet lion on the main deck, to nobody’s pleasure. Even more of a frightening sight is eccentric scholar Dr. Cardigan, George Zucco, who claims to have kept a human brain alive in formaldehyde for six months and is always on the lookout for new subjects.
It develops that a suitcase with $300,000 is missing, the cash tied to a tug‑of‑war divorce settlement between Mrs. Carol Wayne, Claire Dodd, and the murder victim. Carol Wayne is found strangled to death in Judy Hayes’ cabin, where the dead woman and Dr. Cardigan had argued shortly before. Criminal Johnny McCoy, Marc Lawrence, is allegedly being extradited to the USA in the custody of Detective Joe Arnold, Richard Lane, but Chan exposes them as fellow criminals wanted for murder in Shanghai who were trying to sneak back into the USA.
Jimmy starts a “controlled fire” on the ship, hoping to panic the killer into grabbing the hidden money and making a run for it. Charlie’s anger is cooled when Jimmy shoves him out of the way of an assassin’s bullet. The handgun ends up in Hogan’s hands, who returns it to Chan. Chan announces the murderer will be revealed by the fingerprints on the gun. The lights go out and the gun is missing when they come back on, but in stealing the gun, the guilty party triggers an infra‑red camera that reveals his guilt beyond all doubt.
We close with the remaining passengers gathered around the ship‑to‑shore phone, listening to the coos and cries of Charlie Chan’s first grandchild, a hopeful sound that punctuates the detective’s steady, methodical triumph over a shipboard conspiracy.
