The Deadly Affair (Sidney Lumet, 1966)

After Charles Dobbs, a security officer, has a friendly chat with Samuel Fennan from the Foreign Office, the man commits suicide. An anonymous typed letter had been received accusing Fennan of being a Communist during his days at Oxford and their chat while walking in the park was quite amiable. Senior officials want the whole thing swept under the rug and are pleased to leave it as a suicide. Dobbs isn't at all sure as there are a number of anomalies that simply can't be explained away. Dobbs is also having trouble at home with his errant wife, whom he very much loves, having frequent affairs. He's also pleased to see an old friend, Dieter Frey, who he recruited after the war. With the assistance of a colleague and a retired policeman, Dobbs tries to piece together just who is the spy and who in fact assassinated Fennan.