Spymaster Colonel Cairns (Felix Aylmer) already knows something dodgy is going on as photographs of the plans have already been found on captured/killed foreign agents...
So who is the spy? There are a number of suspects and suspicious activities going on but also a few red herrings. The film is a bit slow at times though the suspense keeps you interested and it builds and builds as the film progresses. A satisfactory ending if not a happy landing.
