Within the city of the great pit, lives a cheerful young girl living in an orphanage. This orphanage takes in children whose parents have been lost in the great pit, called the Abyss. This orphanage also salvages in the great pit, serving as an organization that provides professional training to the children. Every child living in these quarters, without exceptions, are fated to delve into the great pit. Contrary to their grim fate, Riko shows a keen interest to the great pit. She happily lives with a robot she finds while salvaging. Then on one day, a message arrives from an explorer who dove into the abyss 10 years ago, never to be found or heard again; it's a message from Riko's mother. And thus, Riko's fate moves forward.