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.
John Bentley
as Det. Insp. Dan Barker
Phyllis Kirk
as Judy Roberts
Leonard White
as Det. Sgt. Mack McLeod
Glyn Houston
as Charlie Williamson
Patrick Jordan
as Albert Fisher, called both "Al" and "Bert"
Harold Ayer
as Steward Alfred Gordon, also called "Al"
David Hurst
as Paddy Maclure
Charles Lloyd Pack
as John Hendrick
Robert Ayres
as Ship's Captain Watford
Ewan Roberts
as Customs Chief J. S. Blake
Michael Balfour
as Adams
