Summer Wars

A shy high school student and mathematics prodigy is invited by a classmate to work at her family home in Nagano during the summer. The girl's family, an extended clan with roots in the Muromachi era, is gathering to honor the 90th birthday of their energetic matriarch. The student learns his "job" involves pretending to be the girl's fiancé for the celebration. While maintaining the charade, he receives a complex math problem on his phone, which he solves out of curiosity. The solution triggers an unexpected event in which an alternate world merges with Earth. He and the girl's family must work together to restore normalcy.