It Lives by Night, also known as The Bat People, is a horror film from 1974 that's best known for its appearance on Mystery Science Theater 3000. It's the story of a researcher who suffers a case of The Stupids and wanders off trail in a cave. Of course, he gets bitten by a bat, and from then on periodically turns into a sort of half-bat-half-human monster. For the Mystery Science Theater 3000 episode see here .