Yousuke Yamamoto's life has spiraled downward since the death of a close friend, turning into a string of monotonous days he endures without drive. On a freezing night, Yousuke encounters a girl named Eri Yukisaki—only for an abnormally tall, chainsaw-wielding figure to suddenly appear and begin attacking her. Despite the terrifying sight, Yousuke is captivated by watching Eri fight and becomes eager to assist her. Nearly killed in the process, he is fortunately saved by the girl as the man flees. Yousuke discovers that Eri is a senior at his school, and while she does not know the man's identity, she can sense where he will appear at night and has gained extraordinary strength to counter him. Due to her newfound powers, Eri feels it is her duty to stop the man from hurting others. Having finally regained interest in life, Yousuke pesters Eri to make him her partner, which she eventually agrees to. Although Yousuke does not prove to be very useful, Eri is happy to have him by her side. As their bond grows stronger, the eccentric pair gradually come closer to defeating the man—however, it seems Eri is still hiding a significant secret.