Saeki Chise has no recollection of the summer of three years ago. Whenever she feels as though her memories might be returning, her head begins to ache and Chise stops trying to remember. That's when Tamiya appears before her. He has a familiar face and photographs from the traumatic time that Chise's mind has suppressed. He uses the photos to blackmail her into becoming his woman... but then Chise is saved from Tamiya by Katsuragi Kaoru, the biggest delinquent in class. Chise begins to fall for Kaoru, but his role in this situation is not as innocent as Chise at first believes...

Summary from animenewsnetwork.com