In The Shack, Willie tells the story of his friend Mack's experience meeting God face to face.
Mack has had a troubled childhood; due to his father's alcoholism and aggression, Mack left home at 13. This experience leaves him with little faith in God. However, in his twenties he settles down in Oregon with a woman named Nan, who has such devout and personal faith in God that she calls him Papa. Mack and Nan have five children: Jon, Tyler, Josh, Kate, and Missy. One summer, Mack decides to take Josh, Kate, and Missy on a camping trip. They stay at a campground for a few days, and meet another family and a couple who are camping in the same area. On the last morning of their trip, Josh and Kate go out canoeing and flip their canoe. While Mack swims out to save Josh, Missy is abducted. When Mack cannot find Missy after rescuing Josh, the police are called and an investigation is launched. The recovery of a ladybug-shaped pin with five dots at the camp site ties Missy's abduction to a serial killer called the Little Ladykiller, and the dress she was wearing on the day of her abduction is found at a shack hidden in the woods nearby. After weeks of searching, no body is recovered.
Mack's family goes on with their lives, though Mack experiences what he calls The Great Sadness, and his daughter Kate becomes more closed off and sullen. One snowy day while Nan, Josh, and Kate are at Nan's sister's house, Mack receives a letter inviting him back to the shack. The letter is signed with the name Papa and has no stamp or return address. Mack decides that it must be either a prank or from Missy's killer, so he decides not to tell Nan. However, he becomes more curious over the next week and decides to go to the shack to confront whatever is there, be it God or a prankster of some kind.
When Mack gets to the shack, there is nobody else there. When Mack sees the faded bloodstain where Missy's dress was found, he starts to cry and smash furniture. He falls asleep on the floor. When he wakes up, he decides to go back home, but after walking a few steps away from the shack, his surroundings suddenly change from snowy winter to warm, sweet-smelling spring. The shack is replaced by a beautiful cabin on the edge of a lake. Inside the shack, he meets a black woman named Papa, an Asian woman named Sarayu, and a Middle Eastern man, Jesus. Together, they are God. Mack spends the weekend at the shack having conversations with the three of them, and in the process he learns to love and trust God. He also works through his guilt and anger at his father and Missy's killer.
At the end of the weekend, he decides to return to his family. While he is driving home, he gets in a car crash. He is unconscious for a few days, and he acknowledges that people may not believe his story about what happened at the shack because of the accident. However, Willie and Nan tell him that they believe him, and the knowledge that Mack gained during his time at the shack allows the police to recover Missy's body and catch the Little Ladykiller.
