When timid eleventh-grader and math genius Kenji Koiso is asked by older student and secret crush Natsuki to come with her to her family's Nagano home for a summer job, he agrees without hesitation.

Natsuki's family, the Jinnouchi clan, dates back to the Muromachi era (1336 to 1573), and they've all come together to celebrate the 90th bi...