A link to the eNotes summary of William Young's novel The Shack is provided below, and should prove helpful. The Shack is a religious story that involves that most horrible of tragedies: the disappearance and murder of a child. Young's theme is the importance of accepting the role of Jesus in one's life under even the most traumatic of circumstances. Indeed, it is precisely under such circumstances that the comforting presence of God is most important.
Mackenzie Allen Phillip takes three of his five children on a trip to the Pacific Northwest (specifically Oregon). Missy, his daughter, disappears, and bits of evidence indicate that she was murdered in a shack in the woods. Young's novel is told in flashback, so we are introduced to Mackenzie ("Mack"), the product of an alcoholic, physically abusive father whose "beat-your-wife-and-then-ask-God-for-forgiveness" connection to religion leaves Mack decidedly agnostic. Four years after Missy's disappearance, with Mack wallowing in the depths of depression, he receives a strange note from "Papa" inviting him to that same shack where his daughter was presumably murdered. Mack goes to Oregon and, upon entering the shack, is introduced to three individuals who the author clearly intends to represent the Holy Trinity. It is through this encounter that Mack recognizes the importance of belief and is reintroduced to the presence of Jesus in his life. The spiritual comfort Mack receives from his meeting in the shack helps restore his faith in humanity.
The Shack is an overtly Christian-themed novel that its author intends to help individuals and families who have experienced traumatic losses cope with their grief and to reconcile the realities of life -- in effect, horrible things happen -- with the presence of an all-powerful God.