The film is based on the actual events of the Portland Spy Ring trial in the U.K. A disgruntled Navy Clerk is transferred to a secret research establishment and is subsequently black-mailed/paid by Czech intelligence to procure secrets for them. He seduces the secretary who controls the most secret documents, and they enjoy the fruits of their treachery until the British authorities begin to close in on them.
Bernard Lee
as Henry Houghton
William Sylvester
as Gordon Lonsdale
Margaret Tyzack
as Elizabeth Gee
David Kossoff
as Peter Kroger
Nancy Nevinson
as Helen Kroger
Thorley Walters
as Cmdr. Winters
Gillian Lewis
as Marjorie Shaw
Newton Blick
as P.O. Meadows
Philip Latham
as Captain Ray
Cyril Chamberlain
as Anderson
Justine Lord
as Christina
Patrick Barr
as Captain Warner
