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.

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)

An alternate retelling of the manga. 
It depicts most of the original characters from the anime/manga accompanied by a handful of entirely new ones, but with the same task of piloting Zearth to protect the world at the expense of their own lives. Most of the regular characters also suffered changes in their appearance and background.

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