The inhabitants of a certain wealthy town think nothing of throwing objects away. However, their waste is priceless to Rudo, a resident of the town's slums. Despite the constant warnings from his adoptive father Regto, Rudo spends his days searching for reusable materials that would otherwise be sent to the giant disposal area known as the Pit. Due to its vastness, the Pit doubles as a means of criminal punishment; those dropped in are never to return again.
When Regto is murdered by a mysterious assailant, Rudo is falsely accused of the crime and thrown into the Pit. To his surprise, he awakens in a trash-filled area inhabited by enormous monsters formed from the junk. As the toxic air and Trash Beasts push Rudo to the brink of death, he is saved by Enjin, one of the Cleaners who wield weapons known as Vital Instruments to fight the monstrosities. Having gained his own Vital Instrument, Rudo soon joins the Cleaners in the hopes of finding a way to escape the Pit and avenge his father.
