When timid eleventh‑grader and math prodigy Kenji Koiso is asked by older student and secret crush Natsuki to accompany her to her family’s home in Nagano for a summer job, he agrees without hesitation. Natsuki’s family, the Jinnouchi clan, traces its lineage back to the Muromachi era (1336–1573), and they gather to celebrate the 90th birthday of the spirited matriarch Sakae. It is then that Kenji learns his “summer job” is to pose as Natsuki’s fiancé and dance with her at the birthday celebration. While trying to keep up with this act around her family, he receives a mysterious math problem on his cell phone, which, as a math genius, he cannot resist solving. As it turns…