During a summer camp, 15 children, 8 males and 7 females, find a grotto by the sea. Deep within they discover working computers and some electronic equipment, and later the owner, a man called Kokopelli. Kokopelli claimed to be a programmer working on a brand new game, in which a large robot has to defend the Earth against alien invasions. He persuades the children to test the game and sign a contract. All but one of them signs, barely a moment later they mysteriously awaken on the shore believing what happened was just a dream. 
(Source: Anime News Network)

One summer, fifteen kids innocently wander into a nearby seaside cave. There they meet a strange man who invites them to play an exciting new video game. This game, he explains, pits a lone giant robot against a horde of alien invaders. To play the game all they have to do is sign a simple contract. The game stops being fun when the kids find out the true purpose of their pact.
 
(Source: Viz Media)
 
The extra story Bokurano: Mou Hitotsu no Bokurano (Yet Another Bokurano) ran after the completion of the main story.

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