A little man in a yellow shirt stands at the door and hands patrons a card which reads: "I will not by my own free will or under duress reveal the ending of Diabolique." Don't sign it.

Even the hocus-pocus of Madison Avenue wags cannot conceal the charm of this seething French thriller. Forget the yellow shirt and the unsigned promise. In the vein of a sardonic O. Henry, Diabolique sometimes is ghoulish and gross, and is never very subtle. The ending, quite as startling as the man in the yellow shirt had you believe, induces a feeling of mental ineptitude. You wonder whether you weren't paying attention at the critical moment; perhaps it's because the director, Henri-Georges Clouzot, is simply a very clever man. He is.

Clouzot has taken a rather average situation and applied a magnificent twist. The deceitful and domineering husband has wife trouble. He openly flirts with his mistress, a teacher in a boarding school of which he is headmaster. His wife has money and she wants a divorce. This would be a bad thing. Propriety, and Madison Avenue, forbid further detail here.

Structurally, the plot seems as perfect as the crime. The film's intensity is slow to generate; interest is sustained, though perhaps not as much as it might, by concentration on the emotion of the distraught wife. Some scenes are grotesque, but they are never offensively so. Paul Meurisse, brutal and dynamic, plays the lecher of women and money. Vera Clouzot, palpitating in guilt and disease, is morally both noble and weak as his wife. Simone Signoret, a Shelley Winters of the Champs Elysees, is calm and ecstatically vengeful. The composite is queer, probing, and quite perfect.