British diamond smugglers use the radiowoman (Phyllis Kirk) from a freighter docked on the Thames to unwittingly "mule" their swag, hidden in cigarettes, past customs, not knowing that she has befriended an Inspector for the Thames River Patrol.
Customs Chief J. S. Blake
Det. Insp. Dan Barker
Albert Fisher, called both "Al" and "Bert"
Ship's Captain Watford