After the murder of Luna, the robot society that rules the planet starts to rust and decay. Death and despair spread through the land like wildfire. The only one unaffected by what becomes known as "The Ruin" is Luna's assassin, Casshern. Unable to remember if he really triggered this capital sin against the entire mechanized civilization, Casshern embarks on a journey to unravel the mystery that connects him, Luna, and the plague.