Susan was raped as a child, resulting in a mental breakdown. She has repressed all memory of the attack, and her assailant is never caught. After being institutionalized for years, she returns to their country estate as a young adult. Her father died, and her mother (Barbara Stanwyck) re-married to "Uncle Harold", a creepy alcoholic. While home, she notices someone following her. She keeps seeing a dead Harold, but the body disappears when she goes for help. Her mother brings in Dr Lomas (Roddy McDowell) to monitor Susan.
Susan gets chased through the woods by an unknown man, to the playhouse where the rape occurred. She shoots and kills the man. It is Harold.
It was all a plot by the mother, with the kind old gardener. They had a mannequin that looked like Harold, which they planted and removed. The gardener chased Susan to the playhouse, and the mother sent Harold there to look for her. The mother envied the love and attention Susan got from the father. The father left all his money in a trust for Susan, and the jealous mother is doing this to keep the money. John, the gardener, is the one who raped Susan.
The mother starts getting calls and visits from the dead Harold. When Harold's corpse chases her through the house, she mistakenly shoots John, the gardener. Dr Lomas and Susan show up with the police. We learn that Harold and Dr Lomas figured out the plot beforehand, when John planted a gun for Susan to find at the playhouse. They enlisted Susan to carry out a double plot. Susan shot Harold with blanks. Dr Lomas certified his death and removed the "corpse". Harold frightened the mother into revealing the plot, otherwise they would have no evidence for the police. John, the rapist, being shot and killed was a bonus they did not anticpate.
Thanks Joseph C!