After the decline of the once robust fields of science and technology, a new energy source known as magic fuels the modern era. Humanity creates forbidden biological weapons: Gears. Those Gears eventually stand against humanity in a rebellion. Although they manage to overcome in the struggle for survival known as the Crusades, humanity's losses are so great that even after several decades , their emotional wounds remain unable to fully heal. Sin Kiske, the child of a human and a Gear, heads to his father Ky and mother Dizzy's wedding ceremony. Their wedding breaks the ultimate taboo: a union between a human and a Gear. Even with the world at peace, it took many years for such a ceremony to come to fruition. Despite the complex array of emotions surrounding it, the wedding ceremony is met with blessings. When suddenly, a mysterious girl appears... The fated child inheriting Gear blood–and the mysterious girl who despises Gears. Their meeting would shake the world.