When British Intelligence discovers that an Iron Curtian country is developing insects as weapons they persuade eminent entomologist Frances Gray to get into the country to collect some specimens. On arrival her cover is almost immediately blown and her contact murdered. The future looks grim for her and also perhaps for the world.
- Actor Character
- Margaret Lockwood Frances Gray
- Dane Clark Bill Casey
- Marius Goring Commandant Anton Razinski
- Naunton Wayne Mr. Hedgerley
- Wilfrid Hyde-White Mr. Luke, British consul (as Wilfrid Hyde White)
- Eugene Deckers Alf, the 'contact'
- Olaf Pooley Detective-Interrogator
- Ernest Butcher Tom, lab assistant
- Gladys Henson Attendant
- Paul Hardtmuth Priest
- Michael Hordern Lab Director Owens
- George Benson Sandwich Stand Customer
- Eric Pohlmann Joe, the bartender
- John Horsley Customs Officer
- Patric Doonan Customs Man
- Joan Haythorne Judy
- Jill Balcon Wardress
- Anton Diffring Officer At Station Check Point.
- John Gabriel Night Field Telephone Officer
- Noel Johnson Frank Conway, Voice
- Anthony Newley Operator
- Lance Secretan
- John Boxer Police Sgt At Customs Cafe
- Robert Cawdron Soldier At Barrier During Fire
- Toni Frost Stewardess
- John G. Heller Soldier At Station Check Point
- Michael Rittermann The 'Tail'
- Ewen Solon Enemy Scientist