Phew. Don't scare me like that. I guess you're just a little sister.

Daimyouzamurai Mei, a mysterious transfer student, enters Hatagaya Academy class 1-F. It seems she has the same ability to see flags and manipulate them, just like Souta. Meanwhile, class 1-F is split into two groups based on their grades into swimming, and go on a trip to the ocean and the forest. For some reason, Souta's forced to join both trips...