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.
| Actor | Role |
|---|---|
 John BentleyDet. Insp. Dan Barker | Det. Insp. Dan Barker |
 Phyllis KirkJudy Roberts | Judy Roberts |
| LLeonard WhiteDet. Sgt. Mack McLeod | Det. Sgt. Mack McLeod |
 Glyn HoustonCharlie Williamson | Charlie Williamson |
 Patrick JordanAlbert Fisher, called both "Al" and "Bert" | Albert Fisher, called both "Al" and "Bert" |
| HHarold AyerSteward Alfred Gordon, also called "Al" | Steward Alfred Gordon, also called "Al" |
 David HurstPaddy Maclure | Paddy Maclure |
 Charles Lloyd PackJohn Hendrick | John Hendrick |
 Robert AyresShip's Captain Watford | Ship's Captain Watford |
 Ewan RobertsCustoms Chief J. S. Blake | Customs Chief J. S. Blake |
 Michael BalfourAdams | Adams |