Long ago, on Earth, a hole was discovered that extended deep into the planet. It was named the Abyss. There live dangerous creatures, and precious relics from ancient times are kept there. The girl Riko dreams of becoming the brave explorer of the Abyss, as her late mother once was. But for now Riko is only one of many junior assistants who perform heavy work on the very top level. One day Riko meets a boy who turns out to be a robot and does not remember how he ended up here. Together they embark on a perilous journey to the very bottom of the Abyss.