College student JIMMY heads to campus with his dog, while in a classroom MADELEINE, the daughter of the UNIVERSITY PRESIDENT, gets a plaster cast made of her face by fiance DR. DONALD BLAKE. They are interrupted as Jimmy shows up with a special delivery - an ancient fish called a coelacanth that Donald has ordered from Madagascar. Donald notes that the coelacanth has a special ability to "resist evolution." The dog drinks some of the coelacanth blood - and soon goes crazy, attacking Madeleine. Jimmy and Donald subdue the dog and stick it in a cage.
Donald takes a sample of the dog's saliva for a rabies test, and notes that the dog has huge teeth. It seems to be an evolutionary throwback to a prehistoric wolf. The UNIVERSITY DOCTOR's assistant MOLLY shows up to get the saliva sample. Donald cuts himself on the coelacanth's teeth as he puts it away, and soon feels woozy. Molly puts him in her car and takes him home. Donald passes out in the car. Molly goes inside to call the Doctor - and a CREATURE comes in and attacks her!
Madeleine and a SECURITY GUARD look for Donald in the lab. He's not there, but the dog is back to being a friendly pet. Madeleine heads to Donald's house. It has been trashed. She finds Donald passed out in the back yard - and Molly dead! Cops soon arrive. At first they suspect Donald because they find his tie clip in Molly's hand, but he is quickly exonerated when the cops find an enormous handprint on a window. Is there a killer on the loose? The cops also note that Molly didn't die of any wounds - she had a heart attack. She died of fright!
Donald lectures to his students about evolution using the coelacanth as an example. Jimmy shows up to check on the dog, and Donald tries to show him the huge teeth - but they are gone. Donald wonders if he is imagining things. Cop STEVENS is sure that someone is personally after Donald, but Donald has no enemies. Stevens gives Donald bodyguard EDDIE.
Donald and Eddie note a small dragonfly land on the coelacanth. Donald also sees "crystallized bacteria" on a slide. He quickly shows the slide to the Doctor, but the bacteria seem totally normal. Did Donald imagine something again? Meanwhile, Jimmy and his girlfriend SYLVIA hear a strange, loud buzzing sound as they hide under some trees to make out. There is a moment of a fake scare, but it turns out to be another couple making out. Jimmy and Sylvia go to the science building - and here they and Donald all see a huge GIANT DRAGONFLY outside the window. Jimmy and Donald trap the giant dragonfly, which Donald recognizes as a prehistoric species.
Donald studies the giant dragonfly. As he does so, blood falls into his pipe, which he then smokes. As the dragonfly shrinks back to its original state, Donald transforms into a PREHUMAN ANTHROPOID! The "anthropoid" smashes up the lab and kills Eddie. The cops come, but find only dead Eddie, a huge footprint, and Donald passed out. Donald recognizes the footprint as coming from a prehistoric subhuman, but Stevens thinks it must have been faked to frame Donald.
Donald investigates, does experiments, and calls a PROFESSOR in Madagascar who investigates how the fish was preserved. Donald finds that the fish was preserved using gamma ray radiation! Donald cancels lots of classes to continue his studies, and Madeleine and her father are afraid he is cracking up. The President comes to Donald to ask him to take a leave of absence. Donald explains what he has learned - the coelacanth blood, treated with radiation, is transforming creatures into evolutionary throwbacks for short periods of time. Just as he explains this, Donald realizes that he was probably the person who transformed into the monster.
Donald heads up to the University President's cabin, where he prepares to experiment on himself. He plans to inject the coelacanth blood into himself and capture the transformation with cameras he has rigged around the room. Meanwhile, Jimmy and Sylvia tell Madeleine about the giant dragonfly. Donald isn't crazy after all! Madeleine heads up to the cabin to see Donald.
Donald injects himself with the coelacanth blood as Madeleine drives. He transforms into the anthropoid, trashes the cabin, and runs out into the road. Madeleine sees the anthropoid, screams, and crashes her car. A FOREST RANGER races to the scene and sees the anthropoid standing over her. He calls the cops and grabs his gun. The anthropoid carries Madeleine away. The cops trail them, and the ranger shoots the anthropoid in the arm. Madeleine wakes up and runs, the creature pursues and knocks her out again, and then kills the ranger. Madeleine recovers and makes it back to the cabin as the anthropoid transforms back into Donald. The cops meet them here. Donald develops the film from the cameras and sees a shot of the creature. He knows now that "the beast within" himself is the killer. He takes the cops outside, claiming to know where the anthropoid is hiding. As the Doctor looks on, he injects himself with the coelacanth blood one last time, telling the doctor to "watch closely and see evolution in reverse." He transforms into the anthropoid, and the cops shoot him before the Doctor can stop them. The Doctor and the cops watch in amazement as the dead anthropoid turns back into mild-mannered Donald.