Ep. 1: Keita Aono cae en el Mundo Inferior y se une a Hinoki Sai en un robot gigante.

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Despite oversleeping yet again, high school student Keita Aono has for once managed to escape punishment by beating his teacher to class. He's also just in time to witness the arrival of an attractive new student. The other boys greet Hinoki Sai with delight, but Keita is too busy remembering her from the strange dream he had that morning. More importantly, he recognizes her as a long-lost childhood friend. His attempts to reminisce with her after class are foiled, however, first by the crowd that's gathered around her, and then by her sudden disappearance into an armored vehicle owned by Akamatsu Industries. Keita's string of bad luck only continues after school, as he's caught in a downpour and loses control of his scooter, plummeting down a flight of stairs into a new underground amusement park called Bottom World. Unfortunately, Bottom World's grand opening has been delayed by the deaths of 200 workers earlier that day, and Keita's clumsiness means he'll probably share their fate. His attempts to escape only lead him deeper into the park -- and deeper into trouble -- as he's pursued by monster after monster. Just when the end looks near, he comes across a giant robot operated by none other than Hinoki! Climbing aboard, Keita takes the place of her missing co-pilot and the two bravely fight off all comers until finally running out of fuel just before facing their most frightening challenge... (Source: GaoGaiGar-Betterman Wiki)