From Derek Winnert's review:

Eric Portman stars as a pathologically jealous businessman called Lee Warren, who thinks that he has committed the perfect murder when he bumps off his devious unfaithful wife Vivien (Greta Gynt)’s latest lover, lawyer Richard Fenton (Dennis Price), and makes it look like suicide after getting him to write her a farewell letter.

But then he finds out she has another lover as well, Jimmy Martin (Maxwell Reed), so he has to change his plans and frame Martin for the crime, making it seem that Martin killed Fenton.

An excellent, creepy performance by Portman, who does menacing and sinister brilliantly, strongly helps to sustain this well-plotted, involving and satisfyingly complicated thriller, based on the popular 1946 London West End hit play by St John Leigh Clowes. It is also a very good showcase role for Greta Gynt as the wife Vivien.

Of course, this claustrophobic movie is as far from real life as possible with its totally artificial and impossible plot, but it does the trick of making you believe in it 100 per cent while it lasts.