Three students exploring a cave run into Trog (short for "troglodyte"), who attacks and kills one of them. When the media gets wind of the killing, reporters (one of whom is Rona Newton-John, sister of Olivia Newton-John) try to film the cave-dwelling killer, who runs them off.
Anthropologist Dr. Brockton (Joan Crawford, in an embarrassing swan song to a long and varied career), convinced Trog is the missing link, prevents the cops from killing Trog, who is instead tranquilized and brought back to Dr. Brockton's lab. (Oh, the groans you will groan when Joan Crawford attempt to play catch with the man in the ugly caveman mask!)
Of course, Trog escapes (he has to, or there wouldn't be any more movie) and kills a dog before being captured again. When Dr. Selbourne (Jack May), Dr. Brockton's rival in science, and a land developer named Murdock (Michael Gough) attempt to set Trog loose (in order to screw over Dr. Brockton), Trog kills Murdock, goes on a rampage, and kidnaps a little girl from town.
Holding the army at bay while she attempts to rescue the girl (who's in no danger, being treated quite sweetly by Trog), Dr. Brockton enters the cave and brings the kid out. And then the army uses dynamite to blow the cave, and Trog, to smithereens. The End.