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ぼくらの
暑假，来到自然学校的15名少年少女。在那里，他们遇到了一个自称为“KOKOPERI”（可可贝利）的神秘大叔，邀请和他们开始一场“游戏”。所谓的“游戏”就是操纵巨大机器人来对抗袭击地球的机器人，以为这是一般电脑游戏的15名孩子们定下了契约，但孩子们不知道这个游戏的真正意义，更不知道操纵机器人背后的沉重代价。在一次又一次的战斗中， 孩子们从不知到真相而对这个游戏的激动兴奋，到知道真相后看着同伴慢慢离开的痛苦，看着每次选择驾驶者也许就是自己却无法解除契约时各自的不同表现，展现了人性中最真实的一面——恐慌、愤怒和悲伤，并通过每个孩子独一无二的人生经历暴露出世界的黑暗一面。 而这个所谓的“游戏”的真相就是：“我们”的地球和其他的（平行世界）地球战斗着，双方各自驾驶着未知机器人战斗，而失败者的命运则是自己的地球的毁灭。在故事中，地球、人类乃至地球上的所有生物都有可能在48小时内完全消失。被选中的15名少男少女为了解救地球献出了他们的生命。但是人类同胞们却对此展现了冷血与无情。而事前定下契约的人，一旦选择迎战后，直到战斗结束为止都不可以退出。更残酷的是他们所操纵的机体活动的能源就是机师的生命，不论胜负，战斗的结果只有死亡。
Fifteen children at a summer school program encounter the mysterious Kokopelli and agree to be testers for a game he claims to have completed recently. Soon, they are summoned into the cockpit of a giant mecha, with Kokopelli piloting. Thus the first round takes place.
The children start having second thoughts about the game, but a floating being known as Dung Beetle appears and teleports them to the cockpit of the mecha, which the group later dubs Zearth. On the evening of the same day, a new enemy appears, and Takeshi Waku is chosen to fight it.
Takami "Komo" Komoda reveals the truth to her father, and while Waku's funeral is being held, Masaru "Kodama" Kodaka is selected to be the third pilot.
Kodama fights the third enemy without caring about his surroundings, causing many casualties in the process, until he discovers the meaning of losing someone important to him.
After the children discover the fate that awaits them, they are assembled and interrogated by the military, except for Isao Kako, who coincidentally is the next chosen pilot.
The children try to find Kako without success, until Chizuru "Chizu" Honda discovers where he is hiding and tries to cheer him up. However, Kako has other plans for her.
The story of Chizu's relationship with her teacher is revealed, before she is selected to replace the deceased Kako.
The fourth battle begins, but Chizu is more worried about taking revenge on her teacher than facing her enemy.
Selected to fight the fifth enemy, Daiichi "Daichi" Yamura's final wish is to spend his remaining time happily with his siblings, but his plans are ruined when the battle starts sooner than he expected.
Bullied by her classmates because of her mother's questionable occupation, Mako "Nakama" Nakarai, after being chosen to engage into the sixth round, resorts to extremes to arrange money enough to complete a personal project of hers in time.
Nagi, a childhood friend of Kunihiko Moji's needs a heart transplant, but so far the only compatible donor found is Moji himself. While dealing with Nagi's drama, he is selected to fight the seventh enemy.
Maki Ano is in awe as her foster mother is about to have a baby. She hopes to at least witness her little brother's birth, but when she is appointed the pilot for the eighth round, she is left with little hope to be able to do so.
During Maki's fight, the children discover the true nature of the enemies they are battling, and what really is at stake on every single bout.
Yosuke Kirie wonders if he will find the resolve to fight in the next battle. While Mitsumi starts to look for means to protect the remaining children, Masamitsu discovers her secret.
The ninth round begins, but after witnessing his mother's miserable life, Kirie refuses to fight. Just when his companions are about to lose hope, something that nobody expected happens.
Given another chance, Kirie finally decides to face the enemy, but when he surprisingly unveils Machi's identity, a commotion is created among the rest of the group.
Chosen to the tenth round, Aiko Tokosumi's life is turned upside down when a scandal involving her father brings up turmoil to her household. But she ends up finding in Kanji the strength to carry on with her duty.
After Komoda was selected for the eleventh match, her father resigns from his position as a politician, but not before revealing the truth about Zearth and the children to the world.
Tamotsu reveals to Jun Ushiro who his real parents were and tells him their story.
Machi reveals to the remaining pilots all she knows about the battles, and how she and her brother got themselves involved on them.
