Yousuke Yamamoto's life has gone downhill following the death of a close friend, becoming a series of monotonous days that he lives through without motivation. On a freezing night, Yousuke crosses paths with a girl named Eri Yukisaki—only for an abnormally tall, chainsaw-wielding individual to suddenly appear and start attacking her. Despite the frightening sight, Yousuke is enthralled to see Eri fight and becomes eager to help her. Almost killed in the process, he is luckily saved by the girl as the man flees.

Yousuke learns that Eri is a senior at his school, and while she is unaware of the man's identity, she can sense where he will appear at night and has obtained extraordinary strength to counter him. Because of her newfound powers, Eri feels that it is her responsibility 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.