The film possesses style, class, elegance, an intriguing mystery, and a pleasant game of lies and half-truths—qualities that suggest it deserves to be considered a great classic. Some people claim this is the best Hitchcock movie that Hitchcock never made, and there’s truth in that statement. The celebrated director wasn't involved in its creation, nor was he connected to the production, but his influence and style are very apparent. The director, Stanley Donan, may have admired the master. Perhaps he did, maybe not—it hardly seems important; the director does a commendable job, and the movie is good. The story revolves around the murder of a man who leaves behind a young wife financially ruined after she’d grown accustomed to a life of luxury. In the days that follow, she learns her husband wasn't who he presented himself to be, and that he was likely killed because of his own greed: apparently, he had taken possession of a large sum of money stolen years earlier by himself and three other accomplices, who will now want their share. Simultaneously, she is contacted by the CIA and a quick-witted gentleman, also potentially unreliable. It's genuinely difficult to know who’s being truthful in this film. The movie has everything needed for greatness, and it frequently appears on television channels dedicated to classic films. Many consider it among the best works of most of the actors involved. The production values are high; there was considerable money invested in the project. Numerous exterior scenes filmed in recognizable locations showcase Paris’s elegance, from its cathedral to the beauty of the Seine River. The cinematography is worthy of attention: while not particularly innovative, it's executed with skill and technical competence. And the music? It’s magnificent. As she often does, Audrey Hepburn delivers a character full of charm and style, her aristocratic bearing accentuated by the costumes, designed by Givenchy (a personal friend). She was at the peak of her career then, enjoying the rewards of success. Alongside her is Cary Grant, offering a familiar face, serious and credible thanks to his restrained appearance: gray hair and a decent look that belie the character’s true nature. They create an excellent contrast: she is young, he is mature; he is intelligent, she is more emotional; he is rationally thoughtful, she is impulsive and lets things unfold; she wants to believe in him, but he offers no real assurances of trustworthiness. The supporting cast is also strong, featuring Walter Matthau, James Coburn, Jacques Marin, and George Kennedy.

Source on TMDB