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.
as Det. Insp. Dan Barker
as Judy Roberts
as Det. Sgt. Mack McLeod
as Charlie Williamson
as Albert Fisher, called both "Al" and "Bert"
as Steward Alfred Gordon, also called "Al"
as Paddy Maclure
as John Hendrick
as Ship's Captain Watford
as Customs Chief J. S. Blake
as Adams
