Summer Wars is a film that fits a lot into two hours: there's action, romance, humor, tragedy, and wisdom. The film tells the story of a young man, Kenji, who agrees to accompany his classmate Natsuki to her grandmother's estate. When he gets there, he discovers that Natsuki plans on passing Kenji off as her fiancee for her grandmother's 90th birthday party. Natsuki's grandmother is a formidable woman who is proud of her samurai ancestry, so poor Kenji will have his work cut out for him if he is to impress her. But after the initial shock of being introduced to Natsuki's large family wears off, he realizes that he's never been part of such a vibrant and charming family before.
This sounds like the setup to a charming, light romantic comedy, and it sort of is, but there's something more to it: Kenji is a math whiz who works on Oz, a sort of mix between Google, World of Warcraft, and AT&T. While he's staying at Natsuki's grandmother's, Kenji receives a mysterious message that was sent to his cell phone asking him to solve a math problem. Kenji solves the problem in a night, then learns in the morning that Oz's supposedly unhackable security system has been compromised. As the intruder takes over more and more of Oz, it becomes a race against time for Kenji to undo the damage and restore Oz before the communications chaos causes permanent harm.
