He fell from the sky

"As I said, you fell down here. I was outside on my daily patrol around the forest and saw you falling. First, I just thought you were a bird that had given up in life or something, but your figure didn't match a suicidal birds."
"Haha very funny" Ivan said sarcastically, and they carried on with their journey.
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.
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Earth. It was a dangerous place now. Monsters and mutated beast roamed the earth's surface and humans had left the planet long ago, being impossible to live on. But one day, a boy somehow finds himself at the planet that humanity had left so long ago. He had no meories of how he arrived to earth, and the onlu thing he knows is that a myserious girl is offering to help him get home. As the journey carries on the two meet great dangers and come over more mysteries about the earth that you could never imagine..