One fateful evening, two twins named Suou and Shion were gazing at the night sky with their father when a falling star fell upon them, turning Shion into a Contractor. Now, two years later, they try to live a normal life. But after unknown men break into their house one day, the duo escapes, Shion vanishes, and Suou finds herself with the masked Contractor Hei. Together, Suou and Hei try to discover why several nefarious syndicates are after Shion, why they’re being chased, and most importantly, what happened on that fateful night two years ago...