Body Parts reminds us that like used cars, bodies don't work well and they break down easily. In this case the mad scientist is a female doctor who offers to replace the arm of a man who has been in an automobile accident. She fails to tell him the arm comes from a condemned killer and that other people have the other arm and the legs. The head has been transplanted and the killer decides he wants hisBody Parts back. The parts on't work well and cause an artist to discover the dark side, and the legs cause a car to run away, and the hero belts around his wife and kids with the killer's arm.
There is no reason given for the experiment and there is nothing to fear; just try not to fall asleep.