Summer Wars (Sama Wozu)

Kenji is a teenage math prodigy recruited by his secret crush Natsuki for the ultimate summer job - passing himself off as Natsuki's boyfriend for four days during her grandmother's 90th birthday celebration. But when Kenji solves a 2,056 digit math riddle sent to his cell phone, he unwittingly breaches the security barricade protecting Oz, a globe-spanning virtual world where millions of people and governments interact through their avatars, handling everything from online shopping and traffic control to national defense and nuclear launch codes. Now a malicious AI program called the Love Machine is hijacking Oz accounts, growing exponentially more powerful and sowing chaos and destruction in its wake.