Bokurano

During summer vacation, fifteen children meet a mysterious man in a seaside cave, surrounded by high-tech gadgets. He says he is developing a video game in which fifteen giant robots defend Earth from alien invaders, and asks the children to test it. They agree, initially pleased by the prospect of playing. The game soon becomes horrifyingly realistic. After the children win their first battle, one of them dies, and they realize they have no idea how to stop the game or escape it.