Hikari Kohinata has gone by herself to beach. To relax, she likes to float underwater and watch the waves lapping onto the shore above her and feel the bubbles surrounding her like she's become one with the ocean. While enjoying herself, she spots a kid sat on the ocean floor staring intently at something. She swims over, and sees that it's a mother octopus inside a rock protecting her eggs. She beckons the kid over for a closer look, and they're in awe.
The two surface and introduce each other, though the kid won't tell Hikari their name. Futaba asks if the kid is local because she's worried that they've been separated by their parents, and the kid becomes offended at being treated like a child. They explain they live nearby and are just cooling off in the sea after tanning. Futaba notes that the kid is already very tanned, but the kid explains that their goal is to become properly tanned like an "ideal man of the sea". The kid changes the subject and asks if Futaba knows more about octopi, because they've never seen one close-up. Futaba explains that mother octopi stay close to their nests to protect their eggs from predators all day and all night without eating or drinking.
A week later, Hikari visits the beach again and spots the kid again, standing out to sea. Hikari approaches them, and tells them that she will help them find the octopus nest again if in exchange for telling her their name. They tell her their name is Kokoro, but that they're embarassed about it because it's a cute name. The two set off for the octopus nest, but spot a huge moray eel that is also en route to the octopus nest. Kokoro busts forward to try and scare the eel away, but it doesn't work and carries on towards the nest. Suddenly the mother octopus bursts out from the nest and constricts itself around it and drags it far away from her nest. Hikari and Kokoro watch it swim away, but Kokoro suddenly realises this means that the octopus nest is unprotected and insists on staying nearby for her to come back no matter how long it takes. After a while the mother octopus comes back, badly injured by its fight with the moral eel but nevertheless goes back to protecting her nest.
Another week later, Kokoro returns to the beach while Hikari is helping her grandmother at her by serving food at the beachside cafe. Kokoro swims off out to sea to find the octopus nest and crosses paths with another moray eel. They try to scare it away by throwing a rock at it, but the rock just slowly floats down. Hikari interevenes by passing infront of the moray and making herself appear bigger by stretching out her arms and staring at it. The eel is spooked away and swims away. Hikari and Kokoro surface, and Hikari explains that although moray eels look menacing, they're actually quite cowardly, and that perhaps in that way they're similar to Kokoro who looks cute but is actually a "reliable man of the sea" on the inside. That makes Kokoro smile.
Kokoro swims back down to check the nest, but notices the octopus is gone. They try to go swim off out to sea to look for her, but Hikari grabs their arm. This angers Kokoro, but Hikari explains that a mother octopus protects her eggs for about a month until her eggs hatch and then she dies from fatigue, and is always destined to die protecting their eggs no matter what. Kokoro begins to cry, and asks what will happen to the eggs if the mother octopus has died, and Hikari tells Kokoro that if she's lucky, they won't be found by predators and will safely hatch and swim out to sea, but if they're not lucky they'll be eaten by predators. Distraught, Kokoro lowers himself down into the water and floats down to the ocean floor, weeping. Hikari notices something surrounding the octopus nest and floats over to Kokoro, tapping their back - suddenly, a whole cloud of baby octopi begins to come out of the nest and swim all around them. Kokoro is extremely moved by the sight. Afterwards, they go to eat tonjiru back at the Amanchu-ya.