[3] It was written by Peter Barnes and Frank Launder based on the real-life case of the Portland spy ring, whose activities prompted "Reds under the bed" scare stories in the British popular press in the early 1960s.

Despite a poor report from his previous superiors, Houghton is posted to the top secret Admiralty Underwater Weapons Establishment at Portland, a Royal Navy equipment testing facility.

[7] Monthly Film Bulletin said "This more or less factual account of events in the Portland spy case has rather the effect of a newspaper serialisation, in which facts and times are carefully recorded, but no one has gone very far with speculation about how the people concerned might actually talk and feel.

Only at the end, with the spies under suspicion, does tension begin to creep in; and only in one scene – Lonsdale’s arrival at the Ruislip villa, with Mrs. Kroger playing the dual role of suburban hostess and secret agent – does the film hit off the mixture of the bizarre and the quietly commonplace (rendezvous in Derry and Toms’ roof garden; arrest in the Waterloo Road) that was obviously aimed at.

[10] David Parkinson in the Radio Times gave it 3 out of 5 stars, and felt "the docudramatic style rather undermines director Robert Tronson's attempts to build suspense," but "Frank Launder proved himself to be just as capable of turning out a nail-biting thriller, as he was of crafting a chortle-worthy comedy.

For once, separated from his usual partner, Sidney Gilliat (although the latter's brother Leslie acted as producer), Launder and co-writer Peter Barnes capably retell the story of the Portland spy ring.