Summer wars

the long version

Kenji is your typical teenage misfit. He's good at math, bad with girls, and spends most of his time hanging out in the all-powerful, online community known as OZ. His second life is the only life he has - until the girl of his dreams, Natsuki, hijacks him for a starring role as a fake fiancé at her family reunion. Things only get stranger from there. A late-night email containing a cryptic mathematic riddle leads to the unleashing of a rogue AI.