Willie, the story’s narrator, introduces his friend Mackenzie Allen Phillips. Mack had a difficult childhood; his father, a violent alcoholic, terrorized Mack and his family. As an adult, Mack is still haunted by those memories, but he has found happiness with his wife Nan and their five children.
The main story opens on a snowy day. Mack watches the snow from his home office, though the beauty of nature mcan’t lift his depression, which he thinks of as The Great Sadness. Mack goes to the mailbox to find a note inviting him up to the shack next weekend, signed “Papa.” It sends Mack a chill of anger and fear. Nan calls to say that she’s worried about Kate, the couple’s oldest daughter, who has been distant. Nan says she is praying to Papa—her nickname for God—for answers.
Three years earlier, Mack took his children Kate, Josh, and Missy—then six—on a camping trip. On the way, the family stopped at Multnomah waterfall, where Mack related a favorite story of Missy’s. In the story, a Native American princess sacrifices herself by jumping off of the waterfall in order to save her tribe. Later that night, Missy tells Mack that the story reminds her of Jesus, and asks if God will ever ask her to jump off a waterfall to save her family. Touched, Mack reassures her that she will never have to do anything like that.
All goes well until the last morning of the trip, when Kate and Josh go to play in a canoe. Missy colors at a picnic table while Mack watches. Kate tries to wave at Mack from the canoe, but it capsizes, trapping Josh below. Mack runs to help his son, but his relief upon saving Josh is quickly overshadowed when he sees that Missy is gone.
A camper tells Mack that he saw a little girl in the back of a truck crying as the truck left the campsite. Police officers arrive on the scene, and one, Tommy Dalton, goes with Mack to look over the campsite. There they find a ladybug pin that matches a signature left by a serial killer known as “Little Ladykiller.” The next day, the truck is spotted near a national forest and Mack joins officers in canvassing the area. Sam asks Mack to identify something in a dilapidated shack. When Mack sees the object, he breaks down: it is Missy’s dress, torn and bloodstained.
In the present, the family has struggled to recover from Missy’s death. Kate has become withdrawn, and Mack feels himself drifting from God. He can’t stop thinking about the note in the mailbox, so when Nan says that she is thinking of taking the kids to see her family, Mack jumps at the chance to go to the shack alone.
Mack borrows Willie’s truck and drives to the forest. Feeling sick and overwhelmed, he enters the dismal shack and breaks down at the sight of the bloodstain on the floor. He leaves the cabin, but the forest starts to transform around him, blooming with the new life of spring. He turns to see the shack transformed into a beautiful cabin beside a lake. As he approaches, the door is swept open by an African American woman who introduces herself as Papa. Behind her is a small Asian woman named Sarayu and a middle Eastern man who introduces himself as Jesus. They explain that, together, they are God, which makes a strange kind of sense to Mack.
Inside, Mack talks with Papa as she prepares dinner. She says she will try to help him understand that he is loved, and that God is always with him. Mack and the three parts of God enjoy dinner together, talking about Mack’s family and the importance of relationships based on respect and love rather than hierarchy and power. After the meal, Papa leads a devotion. Mack is surprised that it does not involve scripture or any formal ritual, but instead is a moment for Jesus to express his sincere love for Papa. After the meal, Jesus explains that Papa is the creator God, Jesus is her son and human incarnate form, and Sarayu is the Holy Spirit.
That night, Mack has a nightmare about Missy. Upon waking he asks Papa if she ever punishes her children, but she says that is not her nature. After breakfast, Sarayu invites Mack out into a beautiful garden, and together they clear a patch of flowers. As they work, Sarayu talks to Mack about good and evil, warning that using subjective measures of right and wrong can lead to unfair judgement of others. It is important to leave judgement to God instead. Mack then goes to join Jesus for a picnic lunch. Jesus invites Mack to walk across the surface of the lake, which startles Mack. But with a little encouragement, he finds that he can easily walk across with Jesus by his side. As they eat, Jesus again emphasizes the danger of hierarchies in relationships. He sends Mack down a path beside a waterfall, saying someone is waiting for him.
Mack follows the path into a large cavern. Inside is a tall, serious woman standing behind a desk. She tells Mack that he is there for judgement—not his own, but to judge God and all humanity. Provoking Mack’s anger by reminding him of his daughter’s killer, the woman asks if God is to blame for what happened. Full of grief and pain, Mack says that God is responsible. The woman tells Mack that if he judges God, he judges all of humanity by extension. She then tries to force Mack to choose only two of his five children to join God in heaven, but he refuses. The woman says that being faced with this choice helps Mack understand Papa. She also explains that horrible things like Missy’s death are not part of God’s plan, but a result of humans choosing to be independent rather than loving and trusting God. The world is broken, and the way to fix it is to give up judgement and independence and trust in God.
Suddenly, Mack hears children’s laughter. One wall of the cavern becomes transparent and he is able to see all of his children, including Missy, playing by the lake with Jesus. Although they cannot see or hear him, Missy seems to know that he is there, and runs over to sign “I love you” and blow him a kiss. As Missy returns to the other children, a waterfall crashes down in front of Mack, obscuring his view.
Walking back to the lake afterwards, Mack realizes that The Great Sadness has lifted. He meets up with Jesus, who explains that the woman in the cave was Sophia, a personification of Papa’s wisdom. Jesus says that he, Papa, and Sarayu were with Missy throughout her ordeal, because God never truly leaves anyone. He also explains that the church as Mack knows it is just a human institution or bureaucracy that gets between Jesus and his believers.
Mack joins Papa on the porch for conversation and some fresh scones. Thinking of the waterfall and the legend of the Multnomah princess, Mack asks if Missy had to die so that Papa could teach him. Papa is disappointed, saying that just because she can create purpose and positivity out of tragedies does not mean that she causes those tragedies. She also says that Mack is afraid of facing his own emotions, which is why he lied to Nan about coming to the shack.
Mulling this over, Mack takes a canoe out on the lake. Sarayu appears in the canoe and invites Mack to return to the cabin for dinner. She tells him that emotions, whether positive or negative, are necessary to experience the full range of what life has to offer. She encourages Mack not to suppress his emotions but to think about their origins. At dinner, Sarayu explains that humans follow rules, laws, and commandments in order to make themselves feel independent and in control, and to judge one another.
After the meal, Sarayu touches Mack’s eyes so that he can see as the three of them see. When he opens his eyes, Mack finds himself on a small hill overlooking a clearing. The world glimmers with light emanating from every living creature. The clearing fills with a group of children glowing with inner white light, and then a circle of adults shining with more complicated colors, and finally a circle of angels glowing blue. One man’s light is volatile. Sarayu explains that the colorful lights are a representation of emotions, and that the man is Mack’s father. Overcome, Mack runs towards him and they embrace and forgive one another. Jesus emerges and individually greets each assembled person.
The next morning, Papa, now appearing as a man with a silver ponytail, shakes Mack awake and makes him breakfast. Sarayu gives Mack a tightly rolled mat full of flowers and herbs from the garden. Mack and Papa begin walking through a path in the woods marked by the sign of a red arc drawn on certain trees and rocks. When they arrive at a clearing, Papa says that Mack must forgive Missy’s killer so that he can let go of his pain. Tearfully, Mack says out loud that he forgives the killer.
Papa then leads Mack into a cave marked by the red arc, where they find Missy’s body under a sheet. Mack wraps her in the scented mat from Sarayu and they head back to the cabin. There, Jesus shows Mack a beautifully carved coffin he has prepared for Missy, decorated with scenes of her with her family. They carefully place Missy inside and take the coffin to the space in the garden that Mack and Sarayu cleared the day before.
Back inside, Papa tells Mack that he has the choice of staying in the cabin, where he will continue learning from God, or returning to his life. Sarayu says that if he returns home, he can choose to keep making the world better by being kind. Mack decides to go back. Sarayu also says that Kate believes she is to blame for Missy’s death, a revelation that Mack hopes will allow him to reconnect with Kate and begin the healing process.
After changing into his old clothes, Mack falls asleep on the floor of the cabin. When he wakes up, he is back in the dilapidated shack, and God is gone. He feels excited to apply the lessons he learned over the weekend. On the drive home, however, another driver slams into Mack’s car. He is airlifted to a nearby hospital and drifts in and out of consciousness over the next several days. Willie visits as Mack is becoming more lucid and asks about the shack. Mack is reminded of all that has happened, and soon tells Nan everything. He calls Kate to his bedside to tell her that Missy’s death is not her fault. Kate is overcome with emotion but also clearly relieved.
After about a month, Mack and Nan go with Tommy Dalton to the area near the shack. Tommy doesn’t believe Mack’s fantastical story until he leads them to the cave where Missy’s body is. Experts descend on the scene, and soon they have enough evidence to find and convict the serial killer who killed Missy. In an afterword, Willie says that Mack has changed dramatically since his weekend at the shack. The Great Sadness has lifted, and Mack is quick to love and forgive. He hopes that everyone can connect with Jesus, Sarayu, and Papa.