Like in Spellbound, the MacGuffin is a trauma locked away in a character's subconsciousness, exposed by certain dramatic indicators (red, storms) in her everyday fears. Her anxiety is sublimated by deviant actions like kleptomania and frigidity. So it's a psychological thriller, adapted from a novel by Winston Graham.
Marnie (Tippi Hedren) marries into old money (Sean Connery) in a romantic melodrama of wish fulfilment gone wrong (like Rebecca). While she gets wealth and status, her suppressed emotions say no. Hedren does a fine job and Connery could have been a great Hitchcock leading man. But this was their only film together. Diane Baker is ideal as Marnie's waspish rival.
The best scene is when Marnie steals from an office safe with a shoe about to fall from her pocket to alert a woman cleaning in the next room. It's a pure expression of Hitchcockian suspense. It's the time-bomb. The passage of the shoe out of the pocket is the ticking clock. But Hitch thought the bomb should never go off as that was bad technique. The shoe falls but the cleaner is deaf...
The big clunking calamity is the matte painting of a ship in harbour in Baltimore which feels so wrong, especially in '64. The back projection on the hunting scene too. Hitch liked to be in the studio. Be warned, the sexual politics are also of their time. It's hokum elevated by the stars and the Master's cinematic know-how.
Sean Connery plays an all-knowing patient psychologist / boss who slowly unravels the cause of trouble in a troubled thief. I was not surprised in the bonus feature explanation to discover that Hitchcock had fused 2 characters to attract Connery to the role. It results in an unbelievable superman/businessman. Tippi Hedren plays a part that Princess Monaco was supposed to play. In short Sean is the only good actor in the cast. Not a bad film just ordinary.