The murders that Mikey was sent to investigate was of two young children and their father. The children's mother had survived and was hospitalized. Evidence pointed to the father as the killer.
As Mikey investigated the murders, he also had to deal with his mentally ill sister, and his memories of his mother and her death.
I have to be honest and fair here...I have read all of Ms. French's book, and while I would recommend all of them, her first, In The Woods, remains my favorite!