Captain Drummond travels to Switzerland in order to marry his girlfriend but the disappearance of a dangerous cargo of explosives makes him delay his plans.

While Capt. Hugh "Bulldog" Drummond listens in, Sumio Kanda informs Colonel Nielson of Scotland Yard that an attempt will be made to steal the highly explosive substance "hextonite" in transit. The inventor of the substance insists on transporting the substance himself, accompanied by his secretary, Draven Nogais. During the flight, Nogais kills the inventor and parachutes the substance out of the plane. He then removes an already severed hand, places his ring on it, leaves it in the plane and parachutes out of the airplane. Drummond happens to drive by the plane crash and retrieves the case of hextonite and the severed hand before Nogais and his accomplice arrive. Drummond takes the evidence home and from the initials on the suitcase determines the owner, and also surmises that the severed hand is several days old and is probably a plant. Suddenly the lights go out and the suitcase is stolen. Drummond, his servant Tenny, his fiancèe Phyllis Clavering and his friend Algy Longworth all board a train at Victoria Station. Nogais is also on this train disguised as a woman, and has put the hextonite in a bottle of bath crystals. Drummond and Algy find the dead body of Nogais' accomplice and realize that Nogais must be on the train. They meet with Nielson on the Dover Ferry and discover Nogais' disguise. Nogais takes Tenny and Phyllis hostage in their own compartment, but Drummond comes in and takes control. An alarm clock goes off suddenly and in the confusion Nogais escapes. Nielson shoots Nogais before he can set off the explosives, and Algy catches the container of hextonite, thereby saving everyone's lives.