In broad daylight on the busy streets of Ikebukuro, a masked murderer goes on a rampage, slaughtering all who stand in his path—save for Madoka Murasaki, who stands frozen as he passes her by, leaving his lingering gaze emblazoned in her mind.

This killer, dubbed "M" by the police and media, haunts the memories of all who witnessed that bloody day; though he is now in confinement, the survivors hallucinate him at every turn in their daily lives. But when society fumbles to find the culprit behind a new series of bloody deaths, Madoka realizes that she has not just been imagining M's presence. Though his motivations are unknown, it is clear that this is the start of a twisted and ruthless culling of humanity.