A scientific expedition to a remote Mexican fishing community, led by Dr. Rick Torres and Susan Lowry, uncovers unhealthy amounts of radiation in the local waters and a small mutant octopus that can crawl on land, hinting at dangers beyond ordinary biology. The team hopes this discovery will bolster funding, but the reception from the scientific establishment is tepid, leaving Torres to seek alternative support.
To keep the project alive, he forges a deal with Johnny Caruso, a circus owner who sees the mutation as a sensational act and promises backing for their work. Yet before they can secure a proper grant, disaster strikes: the monstrous The Octaman attacks the camp and slaughters the remaining crew, leaving nothing but echoes of the day’s horror.
A few days later, the scientists return to the camp in an RV and find it abandoned. Davido, a young Indian man from the nearby village, explains that a local legend about a being half man and half sea serpent may be real, and offers to guide them to the lake where it is said to reside. In the meantime, Octaman continues its rampage, killing several villagers and leaving the expedition with more questions than answers.
The team discovers another small mutant octopus, and Octaman stalks the camp again, claiming another life before slipping away. Johnny Caruso presses on with his plan to capture the creature for his circus, turning the pursuit into a tangible danger for everyone involved. Octaman returns to the RV, and the tension peaks when Lowry blares the horn to summon help. The rescuers manage to flush the monster out, but the chase shifts to the lake as the creature drags another crew member from a boat.
Back at the RV, Octaman captures Lowry, forcing the team to confront the odds. They blind the beast with flashlights and encircle it with a ring of gasoline, turning the flames into a weapon that deprives the creature of oxygen and renders it unconscious long enough to tranquilize it and trap it under a net. The morning brings a thunderstorm that revives Octaman, allowing it to break free and move toward Lowry, who somehow communicates with the creature and sends it away again.
Davido pushes the group to pursue Octaman into a cavern, chasing it deeper than they anticipated. Though the others hesitate, Davido keeps faith, guiding them through a narrow exit that leads back toward the RV—only to find Octaman waiting inside. To spare her teammates, Lowry sacrifices herself by allowing captivity, while the remaining expedition members arm themselves for a final confrontation.
Determined to end the threat, the survivors close in on the pair. Lowry delivers a close-range shot with a concealed pistol, enabling her escape from the beast’s clutches. The expedition members unleash a volley of shots, and Octaman retreats into the lake, sinking beneath the surface as the survivors reflect on what they have endured and witnessed.
