David Phillips (Patrick Macnee) is running down the darkened streets of London's Limehouse district, pursued by two men with guns. He finds a public phone and puts a call through to Dick Barton (Don Stannard), but before he can report, a shot rings out. Barton must piece together what Phillips found out that got him killed. Phillips had been assigned to protect Professor Mitchell (Percy Walsh) and his new development, a ray capable of exploding any unstable element aboard an aircraft in flight. Mitchell has been targeted for kidnapping by Serge Volkoff (Meinhart Maur), a foreign agent from Eastern Europe, as part of a larger, much more sinister plot to destroy England and cripple Western Europe. Complicating matters further is that Mitchell's daughter (Joyce Linden) has also been kidnapped, and Barton must contend with Volkoff's crafty female companion Anna (Tamara Desni).

Third and final film in the 'Inspector Hornleigh’ series of comedy-thrillers. Inspector Hornleigh (Gordon Harker), disappointed at not being handed an important spy case, is assigned by Scotland Yard to an army barracks to investigate the mundane thefts of supplies from the stores. This accidentally leads Hornleigh and Sergeant Bingham (Alastair Sim) to a nest of fifth columnists when his dim-witted assistant carelessly talks to a girl in the cafeteria – and that night, news of Hornleigh and Bingham’s arrival is embarrassingly transmitted back to Germany.