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.
Henry Houghton

Gordon Lonsdale

Elizabeth Gee

Peter Kroger

Helen Kroger

Cmdr. Winters

Marjorie Shaw
P.O. Meadows
Captain Ray

Anderson

Christina
Captain Warner

Chief Supt. Croft

Supt. Woods
Johnnie (uncredited)

Colonel Monat (uncredited)

Police Officer (uncredited)

Surveillance Man (uncredited)

Russian Embassy Official (uncredited)
First Member at Lords (uncredited)

Second Member at Lords (uncredited)

Portland Official (uncredited)

Blake (uncredited)
News Vendor (uncredited)

Tilly (uncredited)