Britain has been suffering from a spate of damaging sabotage attacks and it falls to a trio of skilful men - "Brennan" (Liam Redmond), "Folland" (André Morell) and "Elliott" (Anthony Bushell) to combine police, military and intelligence resources and find out just who is behind these highly co-ordinated incidents. Roy Boulting uses these three accomplished actors alongside some other British household names - Joan Hickson, Dora Bryan, Laurence Naismith, a strong contribution from Mary Morris ("Braun") and a peculiar, but effective, role for the normally arch-Brit Geoffrey Keen ("Williams") as well as quite a taut script and pace, to build this well into a tale of treachery and conspiracy that reaches the upper echelons of the political establishment. The ending is good, if a bit rushed, and Redmond and Morell deliver strongly, keeping the intrigue going, managing their frustrations and keeping the story interesting right til the end - with nobody knowing just whom they can trust. Not seen so often nowadays, but if you like the genre then this is certainly at the better end of these post-war fifth columnist dramas.