Summer Wars

Kenji, a high-schooler good at math, is brought to a big family's house in Nagano to pose as Natsuki's fiancé. One night he solves a problem that arrives on his phone, not knowing it is the cipher for OZ, the virtual world all of the world's infrastructure runs on. With OZ hijacked and daily life failing around them, Kenji is blamed for it and joins the Jinnouchi family in fighting back.