Everyone will eventually see this, or has already seen it in some codified form. Many films are about this one, or about the idea inherent in it.
This idea is to have one or many characters be creators of the story you watch. It is common today because screenwriters have integrated (or been taught) the notion that the world on screen is partially created by the audience. Characters don’t come from life, but from a whimsy desire the audience has to see them captured by the forces of story. Here, that role is acknowledged by having characters in the story be creators of it.
The main character gets entangled into a story that has been created, unbeknownst to him, by some other characters. He falls in love with a woman (who is putting him on), and he almost witnesses her suicide. That suicide is staged of course, but he doesn’t know, and believes her dead. A few years later, he meets her again but she pretends to be someone else, her real self, which he never knew about. He becomes obsessed with her and ‘directs’ her into being his replacement lover, down to costume, manners, and events. He eventually recreates the moment where he almost witnessed her suicide, and at the last minute gets clued into the truth of the situation. He finds out that she never died, she pretended to be someone else who had really died (was pushed), and she simply served as a stand-in for a murder covered-up as a suicide.
When he discovers the ‘truth’, we assume real-life, and real-relationship can begin. But no, she slips, falls, and dies. Is there no escape from the forces of ‘story’? (ha ha.)
The intial ‘story’, besides being a construction to fool the detective, is also a reconstruction of a previous story. It concerns a woman who pretends to live the life of a character from a famous painting.
This is rich storytelling. There is a second-rate love interest in the story who is a painter. She literally creates an image that is an image of the first constructed story, and offers it to the detective. He remains clueless (what a dope).
The filmmaking is precise, never too elaborate. I guess that’s what Hitchcock is famous for. But it is nothing life-altering. The story is why this is important.