Unexpectedly, this is also a form of punishment for Marnie for she can't stand to have a man touch her. The honeymoon is a disaster and Marnie is shocked enough to attempt suicide.
Mark then investigates her past with the help of a private detective, thinks he has found the cause of her problems and together with Marnie visits her mother. There he helps her bring out her repressed memories: her mother was a prostitute, who once - during a thunderstorm - was attacked by a customer; small Marnie tried to save her mother by hitting the man with a poker, accidentally killing him. And, apparently, in this way he sets her on the path to healing.
Marnie is clearly not one of Hitchcock's strongest efforts. It came after the glorious sequence of Vertigo (1958), North by Northwest (1959), Psycho (1960) and The Birds (1963) and in contrast to those films, which are all fresh and new, seems a rehash of old themes, such as those from Spellbound and Vertigo. Suddenly, in Marnie Hitchcock's heavy psychologizing looks very old-fashioned. What works in a film from 1945, is a piece of antiquated junk twenty years later.
Today, nobody believes in these easy psychological explanations of repressed human beings. We known humans are more complex than that. Freud, still riding strong in Hitchcock's time, is today completely out. Therefore the mechanism of the film has lost its spring. The story has become rather unconvincing.
There are other things that grate today, so as the easily recognizable back projection whenever the characters ride in a car, or when Marnie rides her horse. This looks like a cheap B-movie, as do the obvious matte paintings - although the image of a huge ship's hull almost entering the narrow street where Marnie's mother lives, is "aptly Freudian" (quoting the film's ideology).
Connery gives a solid performance, although any moment you expect him to ask for a dry martini - these were the years of Dr. No and Goldfinger. Hedren has been called too weak as an actress to carry the weight of this psychological drama on her slim shoulders, but I felt she has the right tone, indeed prim and frigid, but also frightened. Hitchcock was such a strong director that he could make his actors and actresses do anything he needed on screen. If necessary he would grab their face and mold it with his hands into the expression he wanted!
P.S. Hitchcock's cameo: 5 minutes into the film, in the hotel corridor when Marnie walks by.