Like most murder mysteries, the story is highly improbable; nevertheless, the film is still hugely entertaining, thanks in part to plot twists and turns that even Agatha Christie would admire, and to the film's B&W lighting, that renders a noirish, sinister atmosphere.
The first half is interesting and tightly plotted. But the real strength of the film's underlying premise begins at the mid-point plot turn. The second half is riveting, because the tight plot begins to ooze with mystery and suspense. It builds to a final ten minutes that are as frightening as almost any ending in film history; dark interiors, shadows, ominous light at the end of a long hallway, a general absence of sound, a gloved hand, a scream, and an unexpected image. It's the very definition of spine-tingling suspense.
There is a clue to help solve the story's mystery in the film's first ten minutes; but like any good mystery, that clue is very subtle. All the film's acting is excellent, even down to the children actors. And, Simone Signoret is as wonderful here as she is in all of her other movies.
English subtitles require a little more work for viewers who cannot understand the French dialogue; yet, the story, the acting, and the cinematography should more than offset this minor irritation. Background music occurs only during the film's title sequence and closing credits; this general absence of music thus enhances suspense.
Although not strictly speaking a whodunit, "Les Diaboliques" is a classic murder mystery that has earned a well-deserved reputation for setting the standard for cinematic suspense. The story is riveting, and the film is technically well made. More recent films have tried to copy it; but this is the original.