A scientist must contend with a mutant monster of legend while leading an ecological survey in a Latin American fishing village.
The narrator introduces Dr. Rick Torres who is leading an ecological expedition into a Latin America fishing community.
Dr. Rick Torres records a message that blood samples from the local people show evidence that atomic detonations have carried radiation to all parts of the world. Raul hands him the blood samples collected that day and Dr. Torres asks him if there is any chance of error. Rick tells Susan Lowry that the locals have already absorbed 100 times the safe levels of radiation. Mort Stein walks up to Rick with a water sample and shows him a baby Octaman in a bucket. Susan notes that the way the baby Octaman stares back denotes intelligence and Rick mentions the octopus is supposed to be one of the most intelligent species in the ocean. He then suggests they go back to the area Mort found it. They go to the river and Mort lets it go. The baby Octaman starts crawling towards the water and Rick tells Mort to grab it to take back with them to take to the research station in Vallejo to show to Dr. John Willard. Rick and Susan fly to Vallejo. Raul looks at another baby Octaman and calls it Number 2. The Octaman rises from the river and walks towards the camp. As Raul is starting to dissect the second baby Octaman, Octaman attacks and kills him and retrieves the baby Octaman and brings it back to the river.
Dr. Willard is studying fish in an aquarium when Rick shows up with the baby Octaman. Rick shows Dr. Willard the baby Octaman, but it is dead. Dr. Willard doesn't see a connection between the baby Octaman and radiation, which Rick objects to. Dr. Willard then tells Rick that their funds are going to go to more direct studies. Rick asks Dr. Jameson what he thinks, but he thinks the possibilities are endless. Rick asks Willard to consider the mutation is due to radiation, but Willard tells him no. Rick goes to a ranch to see Steve Dodd. Rick tells Steve that Susan recommended him and Steve tells Rick, Susan told him all about him. Steve tells Rick that Johnny Caruso will be interested in helping to catch Octaman as long as there is a circus attraction involved. They go to see Johnny who tells them he recently saw the movie King Kong. He then introduces him to his dogs Jesse and Frank named after the James brothers. Johnny then agrees to help them. They take an RV to the camp site and find it almost deserted. Rick calls out to Mort and Carlos, but no one answers. Rick searches the camp and finds Raul dead.
The next day, Mort tells him he had gone into town while Carlos went to visit his parents. Enrique introduces himself and offers to bury Raul at their church. Rick asks about a local superstition about a half-man, half-sea serpent and Davido tells him his grandmother saw it once and shows a drawing of the creature. Enrique then tells Rick that Davido's grandmother said that Davido's father was taken by the Octaman. Pedro walks up to Enrique and tells him he wont go to where the Octaman is, but Davido tells them, he will take them. Davido takes them to a lake which connects to other lakes. Enrique and Pedro go to the water with a can to collect water. They take a break and lay down and Enrique suggests they go back to camp before it gets dark. As they leave, Enrique forgets his machete. They make it back to camp, but are attacked and killed by the Octaman. At night, the Octaman watches as everyone sits around and talks. Rick tells Mort that the water in the lake exhibits the same radiation as the village and Johnny asks if drinking it will turn a person into a mutant. Davido tells them that there are some baby's in his village born with mutations. They take the RV on a path along the lake. Susan is standing near the lake and tells Rick she is afraid. At night Davido tells Rick he saw another baby Octaman and he and Rick run off. They find a baby Octaman, which suddenly disappears. The next day, Davido tells Rick and Mort he has found another baby Octaman and they follow him out of camp. Susan runs off and Steve follows her. As Johnny is sitting at camp, he is attacked by the Octaman. Carlos runs over to help him and Carlos is killed.
The rest of the group return and find Johnny injured. Rick asks Johnny what he saw, but he tells him he didn't see anything. Rick refuses to believe that an octopod that walks like a man exists, but Davido shows him his drawing again. The Octaman attacks the camp and Susan and Johnny run into the RV. Johnny shoots at the Octaman while Susan bangs on the horn and the Octaman walks away into the lake. Rick, Mort and Steve chase after the Octaman and take their boat out onto the lake. The Octaman attacks the boat and then drags Mort into the water. Rick and Steve rescue him from the water. Susan goes to look for Davido, but can't find him. The Octaman returns to camp and attacks Johnny when he opens the RV door. Susan throws a lantern at the Octaman, but he grabs her and carries her away. Rick and Steve surround the Octaman and shine their flashlights in its face. Rick makes a circle with gasoline around the Octaman and grabs Susan and runs away. The Octaman eventually collapses from the lack of oxygen. Davido runs and helps Mort off of the boat. Rick and Steve put a net around the Octaman and the next day Johnny wants to go back, but Rick insists on staying and doing studies on its natural habitat.
At night Mort brings up the story of the Beauty and Beast to Susan. The Octaman gets loose and Susan tells the Octaman to get back and it listens to her and leaves. Johnny again suggests they leave and Davido tells them of a place it might hide. They break camp and as they are trying to remove a fallen tree across the road, Davido goes searching for the Octaman. He calls to the others when he sees it go inside of a cave. They encounter the Octaman in the cave and as they are chasing it, they are trapped in a rockslide caused by the Octaman. Davido finds a small shaft and he follows it outside. He then crawls back to the others and tells them the shaft leads outside and the others crawl out of the shaft. They make it outside and see they are right at the RV. As Mort opens the door, he is attacked by the Octaman. Steve, Johnny and Davido are also knocked to the ground. Susan calls to the Octaman and it walks to her and picks her up. Steve checks on Johnny and sees that he is dead. As it is taking her towards the water, she pulls out a gun and shoots it in the chest and it drops her. The others then shoot at the Octaman and it walks mortally wounded into the water. Davido then takes the wood carving he made of it and throws it into the water.