A scientific expedition to a remote Mexican fishing community, led by Dr. Rick Torres (Kerwin Mathews) and Susan Lowry (Pier Angeli), discovers unhealthy amounts of radiation in the local waters. They find a small mutant octopus that can crawl on land, and Torres travels back to the States to present his findings, hoping to be granted more funding. Reception from the scientific establishment is lukewarm, so Torres makes a deal with Johnny Caruso, a circus owner who is interested in the bizarre mutation as a carny act. After their departure, a humanoid octopus, Octaman, attacks the camp and slaughters the remaining crew.