Bokurano

As fifteen children at a summer camp enjoy their carefree vacation together, they stumble upon a small cave near the beach. The group finds a mysterious room deep within its caverns, where they meet a strange man who calls himself Kokopelli. The man invites the children to play a game that involves taking turns controlling a robot to protect the Earth against aliens. Thinking it might be an interesting computer game, all but one of the children sign a contract agreeing to play.

Later that day, the giant robot appears in person and the children are teleported into its cockpit, where they find Kokopelli, who pilots a fight against a similar robotic creature that has appeared nearby. After winning the battle, Kokopelli tries to apologize but is interrupted as the children are teleported back to the ground in the midst of the carnage.

As the bewilderment following the fight settles, the robot suddenly disappears and the children are left scared and confused. Despite seeming like a harmless game at first, the children never anticipated the stakes of this game and how their pasts would intertwine with their decisions going forward.

(Source: MAL)