This Island Earth (1955)

Dr. Cal Meacham (Rex Reason), a nuclear engineer, is recruited by Exeter (Jeff Morrow) the leader of a mysterious group to assist them in atomic research. While I thought the expanded foreheads and white hair of Exeter and his assistant would give this guy a clue, apparently he ignores these unusual features in pursuit of scientific knowledge. Things are a little too happy in Happy Valley, so he eventually decides to nose around a bit and discovers that something indeed is up in the Den of Mark. Noteworthy appearance by Russell Johnson (The Professor of Gilligan's Island).