The narrator and protagonist is ten-year-old Aoyama. A serious student, he has already acquired a broad knowledge of the world and the universe, and he keeps notebooks in which he records day-to-day observations from his studies, explorations, and experiments. In May, some Adélie penguins appear in the town where he lives. They are captured and hauled away by truck, but then they vanish into thin air while being transported. Aoyama talks about the penguins with a young woman who works at the dentist’s office and who has been a friend to him. The woman is referred to only as Onee-san. When Onee-san tosses a soda can one day, it turns into a penguin. Meanwhile, Aoyama’s classmate Uchida has secretly been keeping a penguin, but it hasn’t touched its food for three weeks. When Aoyama and Uchida take it with them on the train, it disappears. Another classmate, a girl named Hamamoto who often plays chess with Aoyama, discovers a strange spherical object in a field on the edge of town. The three classmates name the sphere “Ocean,” and begin keeping a log of their observations of it. Aoyama soon concludes that there is a relationship between the swelling and shrinking of “Ocean” and Onee-san’s state of health.