Light comedy thriller about a young lawyer Larry Preston (Barry K. Barnes) and his wife Janet (Diana Churchill), who were married in real life, who specialises in getting suspected 5th columnists off charges, much to the dismay of his boss Alistair Sim (the only bit of comedy in this film); but he has an ulterior motive in hoping to befriend them and infiltrate the gang.
The police are not too keen on this practice, but are frustratingly left one step the lawyer - the police being played by Edward "Mr Grimsdale!!" Chapman and Geoffrey Sumner, who went on to play much the same character of old-school upper-class twit in such things as the Colonel in The Army Game.
Others of note were Leo Genn, Cyril Smith (not the fat politician) and Carl Jaffe, who pops up as a foreign dodgy character in a number of films from this period and whose son entreated us to the life of his father, some time ago on the BCG forum.