15 years ago, a mysterious "Ray Ring" of light appeared, encircling the world, thereafter on occasion dropping mysterious objects called RUMs to the ground, which cause small localized disaster areas. The objects are predicted and the disasters contained, so the RUMs are largely ignored by the public.

Under this incredible backdrop, the ordinary Seita Tooki has just been confessed to by his lifelong enemy, Awayuki Kujou. Before he can give her a proper reply, a RUM unexpectedly descends, surveys the situation, and proceeds to kill him, as well as the entire city nearby as collateral damage. As it turns out, the Ray Ring above is apparently home to 7 billion "Grim Reapers," nearly all of whom seek to deny their newfound love.