A scientific expedition to a remote Mexican fishing community discovers unhealthy amounts of radiation in the local waters. They also find a small mutant octopus with nearly-human eyes that can crawl on land and make mewling sounds like a baby. In order to receive further funding for their project, Dr. Rick Torres travels back to the States to make a deal with circus owner Johnny Caruso, who is interested in the bizarre mutation as a carny act. They return to the camp to discover that their crew has been slaughtered by someone (or something) and the octopus specimen is missing. A young man from the village says that a local legend about a creature said to be half man and half sea serpent is true, and offers to take the scientists to the lake where it is purported to live. What they find is a seven foot tall walking octopus with amazing strength and a lust for killing, and soon the expedition realizes that the monster is now hunting them.