The Lady and the Monster (1944)

Prof. Franz Mueller is trying to sustain brain activity to keep important people's minds beyond their deaths. A fortuitous plane crash allows a Mueller to remove and preserve the victim's brain in a special solution he has been developing. As the brain is treated with plasma, it begins to telepathically take control of the scientist's assistant, Patrick Cory. The longer this continues, the stronger the connection to Patrick's mind. In fact, his whole personality takes on aspects of Henry Donovan, a ruthless millionaire financial wizard. Donovan wants Patrick to win freedom for Roger Collins who has wrongfully imprisoned for a murder he didn't commit. Mueller's other assistant, lovely Janice Farrell, is conflicted. She knows that Mueller has eyes for her, but is repulsed by him as a person. She is sweet on Patrick. The two plan to run away together until Donovan's personality takes over.