Mysterious Ward Threshold

As a child, Amasawa Rei had attended Shirohato Park Institution. After her parents were killed in a car accident, she lived at the institution and befriended a boy. She doesn't remember anything from the time she spent at the institution, not names, not faces, nothing, only that she and the boy shared a secret--a secret much darker than she thinks it is. Now, as a new student at Shirohato Academy with her old classmates from the Park, she is assailed by fragmented memories of the past. What was the secret? Why does she feel driven to find that nameless boy? And most importantly, why doesn't Rei remember anything from her past?