The Two-Headed Spy (1958)

Jack Hawkins wasn't always renowned for the subtlety of his performances, but he does manage that well here. In this quite plausible adaptation of AP Scotland's fact-based book, he portrays a German officer who works his way up the ranks, ultimately to reach staff rank in the Nazi army - whilst passing on secrets to the British for almost 30 years. The direction is suspenseful and it maintains the tension well - especially towards the end. This is rarely seen, nowadays, but is eminently watchable.