It's a Hitchcock film about a husband, a wife and another man. You don't want me to tell you any more. You can work it out from the title.
It's adapted from a Broadway stage play and it stars Ray Milland, Grace Kelly and Robert Cummings. It's set in London, although clearly filmed in America if you listen to the accents of any policeman who only says a couple of lines. The important thing is that it's fun. Everyone's so gentlemanly and well-mannered, you see. It's terribly English. No one shouts or gets stroppy. The more extreme the situation, the funnier it is to see a murderer being suave and graceful about their lot.
It's also a classic example of Hitchcock's "suspense vs. surprise" paradigm, except that it then goes beyond that to the next step. If we're having a chat over dinner and suddenly a bomb blows up under the table, the audience will be surprised. (We won't be, because we're dead.) However if the audience had seen the bomber put it there fifteen minutes earlier, that's suspense instead. Here there's no mystery whatsoever about the murder. The killer explains their plan to us in exhaustive detail. They walk through it. It couldn't be clearer if they'd drawn a diagram. Thirty minutes into the film, we know everything... except what's going to happen afterwards, because we know there's still more than an hour to go.
What we end up seeing is The Adventures Of A Murderer. (I'm tying myself in knots trying to avoid spoilers here.) Our protagonist is a bad person, whose enemies are the forces of good. We see cleverness. We see planning, improvisation and ingenious lies. We hope that the noose will end up closing in on the right person, but for a while it was looking touch-and-go. Obviously this is more entertaining than watching the good guys, although that said I loved John Williams's Chief Inspector Hubbard. He's deliciously sharp, yet also capable of seeming a bit dozy and he has some funny dialogue. "Highly irregular, of course, but my blood was up." "Even I didn't guess that at once. Extraordinary."
The biggest name in the cast is Grace Kelly, of course. It's amazing to think that she retired at the age of 26, having only become a proper star three years earlier with her Oscar-nominated role in Mogambo. The most striking element of her performance here came from Hitchcock, incidentally. He directed her to play certain scenes as if in a trance. It's brilliant.
Meanwhile John Williams is recreating his stage performance, for which he'd won a Tony Award. Also borrowed from the Broadway version is Anthony Dawson as Captain Lesgate. The killer is the key role though, of course, and that's just as you'd want it. (Cary Grant had wanted the role, incidentally, but the studio said audiences wouldn't accept such type-breaking casting. This sounds daft to me and I think he'd have been wonderful.)
It's a 3D movie! This was the "golden era" of 3D, i.e. 1952-1955. Warner Brothers thus insisted on shooting in 3D even though by then it was heading out of fashion. Apparently Hitchcock wanted the opening shot to be of a finger dialling M (for Murder) on a telephone, but 3D cameras weren't then capable of focusing the close-up correctly. Undeterred, he ordered a giant wooden finger and had it turn an outsized dial. The film was eventually released in simultaneous 2D/3D forms and so didn't play in 3D anywhere at all, as Hitchcock had been assuming all along. However apparently it's seen as one of the best examples of the 1950s 3D process, so since then it's had a 3D re-release in the 1980s and a 3D Blu-ray in 2012.
There are remakes, including (loosely) A Perfect Murder in 1998 with Michael Douglas and Gwyneth Paltrow. There are also Russian and Bollywood versions.
This isn't one of Hitchcock's twisted, disorientating descents into darkness. It's fairly straightforward, as you'd probably guess of a stage adaptation. (The original playwright also wrote the screenplay.) It gets darker as it goes on, obviously, which you can even see in Grace Kelly's clothes and in the music, which starts out light-hearted and jolly. However the film also gets funnier. Loved it.