Aoyama is a serious 10-year-old boy who records all of his day-to-day experiences in his notebook. One day in May, penguins inexplicably appear in his home town, despite being located a long way from the sea. When Aoyama sees Big Sis a young woman who works at a dentist s office drop a soft drink can which inexplicably turns into a penguin, he decides to investigate and resolve the mystery behind these strange events.
Together with his classmates, Aoyama happens upon a strange phenomenon which is surely the key to these goings-on. Will their investigation bear fruit, and just how is Big Sis linked to these occurrences?