Ben, a young boy from St. Paul, Minnesota, sits on a dock in a remote northern Minnesota town with his father, David, waiting for his mother, Lindsey, to arrive by canoe. Ben has not seen Lindsey since she left the family when he was three years old. As they wait, he silently reassures his imaginary dog, Sunshine, a small reddish-gold spaniel he invented as a toddler. Sunshine appears whenever Ben thinks of her, though no one else can see her. David has grown impatient with the imaginary companion, believing Ben is too old for it. Ben harbors a secret plan: By spending a week with his mother, he hopes she will get to know him, want him, and decide to come home so the family can reunite.
When Lindsey's canoe reaches the dock, Ben notices her long braid is the same coppery reddish-gold as Sunshine's fur. He recognizes her pale, freckled face despite having no photos of her. Flustered, he greets her with "I'm very glad to meet you," which makes her laugh and embarrasses him. Over lunch at the town's café, Lindsey reveals her island cabin has no electricity, phone, or cell reception. David nearly cancels the visit, but Ben protests, and Lindsey promises to keep him safe. The parents share a long look that convinces Ben his plan will work.
Ben and Lindsey paddle to the island, portaging between two lakes. The cabin is a single-room log structure inherited from Lindsey's grandfather. Ben's only sleeping spot is a high loft reached by a slender ladder. Terrified of heights, he forces himself to climb without looking down.
The next morning, Lindsey asks who Ben was talking to in the loft. He confesses he has a pretend dog named Sunshine, bracing for her to say he is too old. Instead, she calls Sunshine a "daemon," a guardian spirit that takes the form of an animal, and Ben is thrilled. Later, he hears an eerie howl outside, mistakes it for wolves, and runs inside in a panic. Lindsey embraces him and explains the sound is only loons, then laughs at his mistake. Ben storms out, humiliated. He reflects on his many fears and worries that his fierce, wilderness-dwelling mother will never understand a scared child. His father calls him a "what-if kid," and Ben fears the label fits.
Over breakfast, they begin to connect. When Ben asks about her childhood, Lindsey reveals her own mother hit her frequently, alternating violence with affection. Ben asks softly whether she wanted him. She replies he was "the kind of kid any mother would want" (62), but the answer feels general rather than personal, leaving him emptier. On a canoe excursion, Lindsey explains that great blue herons survived because the timid ones avoided hunters, a notion that comforts Ben. In a clearing of marsh marigolds, Lindsey puts her arm around his shoulders. The warmth seeps into him, but involuntarily his shoulder shrugs her hand away. The unconscious rejection haunts him.
Over lunch, Lindsey tells Ben she knows he is angry. He shouts that he is not, but Lindsey says there is nothing wrong with being angry and that he has good reason. Ben offers to spend each afternoon outside so Lindsey can work on her novel, though he mostly sits on a log daydreaming about his plan.
Lindsey takes Ben to a neighboring island to see a mother bear and her cub. When Sunshine creeps toward the cub, Ben shouts at her to stop, alerting the mother bear. Lindsey pops open an umbrella to startle it, but Ben flees to the canoe. Lindsey emerges unharmed, though her tight expression tells Ben he has damaged his plan. Later, she suggests it may be time to examine what Sunshine is "about" and asks if Ben remembers their old neighbor Mrs. Schwartz and Shadow, a Cavalier King Charles spaniel. Lindsey points out that Sunshine matches the breed in coloring, but Ben resists the conversation.
That afternoon, Ben takes the canoe out alone without permission to prove his bravery. A strong gust spins the canoe, and he loses direction. He lands on the wrong island, discovers a bear footprint, and pushes off into the waves again. Exhausted, he eventually reaches his mother's island but cannot find the cabin. He builds a small campfire using matches stashed beneath the canoe's bench, then smothers it before resuming his search. He spots the outhouse through the trees and makes it back, but Lindsey, absorbed in her writing, never noticed he was gone.
When she realizes what happened, she is shaken. Ben asks bitterly whether she would have noticed if he had not returned. She says she would have "broken in two" (140). He demands to know why she left. Her answer is quiet: She left to keep him safe from herself. Ben climbs to the loft in silence. Lying on the sleeping bag, he reaches for Sunshine but cannot find her. The dog has vanished.
That night, Lindsey shakes him awake. The island is on fire. She sends Ben onto the lake in the anchored canoe while she fights the blaze on shore. Alone on the dark water, Ben watches flames crawl across the island, suspecting his campfire is the cause. He calls for Sunshine and spots a reddish-gold glow moving along the shore. He paddles back and follows Sunshine to where Lindsey is digging a firebreak around the cabin. As Sunshine reaches Lindsey, the dog seems to leap into her and vanish. Ben insists on helping, and they work side by side clearing brush from the cabin walls before retreating to the canoe. Ben confesses about the campfire. Lindsey listens, then says only "Well." At dawn, the cabin still stands, though the outhouse has burned. Sunshine has not returned.
They paddle back to town. At the café, Lindsey calls David to pick Ben up early. Angry that his visit is being cut short, Ben blurts out his secret plan: He came hoping Lindsey would want to come home and be a family again. Her expression tells him she will not come. He reveals his lifelong belief that he did something terrible as a toddler to drive her away. He remembers fragments: a red fire truck, a glass of milk shattering. Lindsey tells him the full story. It was an ordinary lunchtime; she had spent the morning cleaning the kitchen to David's exacting standards. Toddler Ben brought his Matchbox fire truck to the table and bumped a glass of milk. She snatched him from his chair and looked at him as though she might hurt him. She saw the terror in his face and knew she could not trust herself not to repeat the cycle of abuse from her own childhood. She carried him to Mrs. Schwartz, where he played with Shadow, the dog that became the model for Sunshine. Then she left and never came back.
Lindsey forgives Ben for the campfire, suggesting lightning was the more likely cause. She asks if he can forgive her. He says he can forgive her "for being tired of cleaning the kitchen" (187). She asks if he can also forgive himself for being three years old, for spilling the milk, for having a mother he could not keep. He nods. She tells him he has not been a what-if kid during this visit: He asked to come, climbed the ladder, slept in the dark loft, and went out alone in the canoe.
When David arrives, Lindsey places her palm over Ben's heart and tells him Sunshine is right there and always will be. As she turns to leave, Ben calls after her and asks to visit next summer. Her face lights up. He runs and throws his arms around her, the embrace he could not give before. Stepping back, he feels Sunshine pressing against his leg. He offers to leave his daemon with his mother for company until he returns. Lindsey accepts, pats her leg, and Sunshine follows her. David wraps an arm around Ben's shoulders. Ben leans in, ready to go home, his heart singing with the promise of next summer.