The Mind Thing is an alien who was unfortunately transported from its own planet to Earth as punishment for a crime that it committed on its own planet. The mind thing is nothing more than a brain in a turtle like shell, but it has the power to take over another mind (human or animal) while that person is asleep. However, it can only leave that mind upon the person or animal's death. It thus can only leave a human's mind by forcing the person to commit suicide. It wants to go home and needs a human to build it a "projector" to get it back home. It takes over many animal and human minds in order to accomplish its goal, but ultimately is stymied by a brilliant professor, who starts piecing together the unusual suicides and puts 2 and 2 together. His secretary, with whom he is not intimately involved, ultimately kills the alien.
This report prepared by speaker43