There is a preface by journalist Alistair Cooke which informs us that what we are about to see is all true! It is adapted from a case study by psychiatrists of a woman with multiple personality disorder. This documentary style approach helps prevent the film from occasionally falling into unintentional comedy.
The real patient actually claimed over twenty personalities. The film gives her three. She challenged this version of events. Of course, this is just screen melodrama. The psychiatrist (Lee J. Cobb) ultimately cures Eve (Joanne Woodward) through some extremely unconvincing Hollywood Freud. But it is fascinating and fabulously entertaining.
The film leans heavily on Woodward's performance. She deservedly won an Oscar. Without her credibility it would be too difficult to suspend disbelief. She plays three working class characters from the southern states. Eve White is a repressed introvert. Eve Black is an extroverted good-time girl. Jane is a kind of balancing superego. Woodward slips with fluidity between each.
It's not a visually impressive film. The director- Nunnally Johnson- was usually a screenwriter, and he tells the story well. There's some comedy when Eve's husband explores the possibilities of being married to three contrasting wives! But any frivolity is balanced by the impassive narration. It does touch on the consequences of mental disability, but this is chiefly offbeat escapism.