Michiru Kita is a timid girl who struggles to voice her thoughts. One day, she spots mysterious, ring-shaped marks around the necks of two classmates, Chika Akatsuki and Shito Tachibana—two boys who miraculously survived a terrible accident six months prior. Michiru, who possesses the rare ability to see these rings, knows they signal impending death. Hoping she might be able to save her classmates' lives, she approaches them... but it appears the boys have already struck a different kind of deal and incurred a heavy debt.