"I will never forget the story of that summer story." Aoyama, a boy in elementary school (fourth grader), learns about the world all day long and records what he learned in his notebook. He is friendly with the Onē-san (sister) at the dental clinic and has a crush on her. One day, a penguin appears in the suburban city where Aoyama lives. It suddenly appeared in a residential area, where there is sea! And then it disappeared! Then, he witnesses a can of cola thrown by Onē- san turning into a penguin. How mysterious! After that, Onē-san asked Aoyam to solve this mystery. So, Aoyama started studying to solve the mystery of penguin and investigating the matter. Where do the penguins come from? Where did they go? What is the relationship between "older sister" and penguin? And can you solve this mystery?