Summer Wars. Volume 1

During the summer holidays, Kenzji receives an offer from his beloved Natsuki-senpai to work, and they head to her great-grandmother’s house. But who knew that the job was to pretend to be Natsuki's fiancé?! He spends the day visiting her large family, and at night he receives a suspicious letter with a math problem. Kenzji finds the answer, not even suspecting what an unprecedented catastrophe this will cause...