During a torrential rainstorm out at sea, two brothers are hanging on desperately in their rowboat as it suddenly capsizes from the waves. The two make their way to a small home and ask the owner if they can stay there for the night to brave the storm. The owner lets them in and suggests that they should change into dry clothes, then goes to find them some from a dresser. As the two begin to undress, a gun drops from the waist of one of them, who quickly hides it in a basket. As the home owner invites them to sit for tea, one of the brothers asks him if he knows of any good divers, to which he replies that he is one. The two tell him that they’ll talk more about what they’d like him to fetch from the ocean tomorrow.
The next day, the three make their way out to where they want him to dive, informing him that it’s a suitcase that he is looking for. As he makes his way down towards the suitcase, he sees Agon resting near it, and immediately swims back towards the surface. He tells the two that he saw a monster that he is certain was Agon, and that the suitcase was right at its foot. As he tells them that he must go to inform his neighbors, the elder brother insists that he keep things quiet until he salvages their suitcase once Agon leaves, and threatens him with his gun that he do what they ask.
Goro and Satsuki are out fishing when Monta arrives and tells them that it’s a terrible place to fish, and goes home to get his fishing rod. When there, he finds his father and the two brothers; the younger one taking him hostage so that his father doesn’t try to escape or alert the police.
The three go out to the salvage area again, but with Agon still resting in the area, Monta’s father is unable to retrieve the suitcase. The two brothers decide that moving Agon would be the only way to salvage the suitcase, and the elder brother suggests using uranium to lure him away, stealing some from the Fuel Laboratory.
That night, the two of them make their way into the laboratory, incapacitating a guard and taking his keys before stealing a uranium fuel rod. The next morning, Yamato and Goro arrive at the laboratory. Monta’s father arrives soon after, telling them that he has information related to the robbery and that the two have his son hostage. He reveals that they took the fuel because Agon was in the area, and Dr. Ukyo quickly reports the monster’s location to the military.
Monta’s father manages to retrieve the suitcase and brings it back to the brothers, who are convinced that he was lying to them about Agon and plan to keep Monta as a hostage until they escape. Monta’s father tries to fight them off, and Goro, having seen him making his way back to shore, arrives to help. As the elder brother pulls his gun on Goro, Yamato quickly appears and disarms him.
During the struggle, Monta was set adrift in his father’s boat, and it begins floating towards Agon, who has surfaced having sensed the uranium. The military begin shelling Agon as it surfaces, but their attacks are unable to injure it. The monster takes the boat and Monta in its mouth, forcing the shelling to cease. The two brothers, realizing the suitcase is in the boat, decide that they’ll need to follow Agon and force it to let the ship go, as the monster, still firmly clutching the boat in its jaws, begins to make its way onto the beach.