Summer Wars (2009) on Netflix

Plot summary

Kenji Koiso is an eleventh grader with a gift for mathematics but little social ease; he spends his spare time as a low-level moderator in OZ, a vast virtual world. When popular classmate Natsuki Shinohara asks him to join her at her family home in Ueda, Nagano, he agrees without knowing the plan. There he is introduced to the gathered relatives as her fiancé, there for her great-grandmother Sakae Jinnouchi's 90th birthday. Among them is Wabisuke Jinnouchi, a software engineer who has lived in the United States for a decade. Then a coded e-mail arrives, and the answer Kenji sends is used to hack OZ from his account.