A college professor by the name of Donald Blake orders a coelecanth for his studies. A student picks it up and delivers it to his office.. However, the container is leaking water and blood and the student's German Shepherd licks some of it. A few minutes later the dog has huge canine teeth and tries to attack them. They manage to subdue the dog and put it in a cage, where it continues to growl at them. Later, when the scientist moves the prehistoric fish to his freezer he cuts his hand on its teeth. He starts feeling sick and transforms, not into Thor but into a primitive ape-man. He ends up frightening a college girl to death. Then a cop is killed and the only thing the authorities have to go on is a footprint of the apeman. It's not until a dragonfly lands on the fish and grows to two feet long that the professor does some research and discovers that the fish was exposed to Gamma rays in order to preserve it. These rays somehow interacted with the fish's plasma and now has the capability of de-evolving creatures.
But it's not until an hour into the movie that the Professor actually starts suspecting himself as the ape-creature. Duh. Who else had access to the fish. He secludes himself at a friend's cabin in the country and injects himself with the fish blood again. He changes again. Later, he tells the police that he can help find the creature--and goes off into the hills and changes again. And they shoot him. He basically kills himself because he can't live with what he's turned into. As the cops look down at the body they wonder where the Professor went off to--until they see him change back into his Dr. Jeckyll persona.
This has a bigger budget than THE NEANDERTHAL MAN, which was made half a dozen years earlier, and the special effects are better.