Jackson Hurd, who goes by Jack, lives with his parents on a farm in Maine. Jack learns that his parents plan to foster a 14-year-old boy named Joseph Brook. Joseph recently attacked a teacher at his juvenile detention home, and he has an infant daughter that he has never met. When Joseph arrives on the Hurd family farm, he is quiet and easily startled.

The boys walk to school together the day after Joseph arrives to the farm. They are late, much to the chagrin of the school’s vice principal, Mr. Canton. Despite this negative first impression, several of the school’s teachers take a liking to Joseph. The next day, the boys stop at an old church and throw stones at the bell to make it ring. Jack learns that Joseph’s daughter is named Jupiter, after his favorite planet.

One morning, Joseph decides to walk out onto the frozen Alliance River. Joseph’s recklessness scares Jack, who once saw a dog fall into the river’s ice and drown. Joseph falls into the river, but Jack saves him, and that night, Jack sees a long scar on Joseph’s side.

The boys start riding the bus to school. Jack’s friends warn him against spending too much time with Joseph after Joseph treats another boy violently. The gym coach, Coach Swieteck, keeps Joseph separated from the other eighth-grade boys.

The Hurds and Joseph skate on a frozen pond, and as they skate together, they ask Joseph to tell them about his past. Joseph explains that when he was younger, he helped his father, Mr. Brook, with plumbing work. He met a girl named Maddie on a job and returned to her house days later. They spent her summer and winter breaks together; during the latter, they had sex for the first time, when Maddie was 13. Maddie became pregnant, and Joseph was moved to a juvenile corrections home. He broke out repeatedly to try to see Maddie. Authorities sent Joseph to a more secure correction home; there, Joseph’s case manager, Mrs. Stroud, asked Joseph to allow his daughter Jupiter to live with adoptive parents. Here, Joseph also learned that Maddie has died, and when he was upset, Joseph accepted drugs from another boy and attacked a teacher.

At school, an eighth-grade boy and his friends attack Joseph in the locker room after gym, and Jack involves himself in the fight. The school suspends Joseph, but several of his teachers visit the farm to tutor him. Joseph attends his first church service with the Hurds on Christmas Eve. He hears the Biblical story of Mary and Joseph, which resonates with his past. On Christmas Day, he receives a note from Mr. and Mrs. Hurd announcing that they will help him find Jupiter.

In the new year, the Hurds learn that Jupiter is in the nearby town of Brunswick, and Joseph runs away to find his daughter. The Hurds look for him, but a severe winter storm stops them. When the weather clears, the Hurds drive to Brunswick. There, Jack meets Jupiter’s foster mother, a librarian. While at work, she receives a call that Joseph is at her home and she agrees to take Jack with her to her house. Police arrive, but the librarian is able to defuse the situation, and she gives Joseph a photo of Jupiter.

The librarian writes letters to Joseph. The boys return home from school one day to find Joseph’s father waiting for Joseph. Mr. Brook threatens to hurt Jack if Joseph doesn’t leave with him. To save Jack from his father, Joseph leaves with Mr. Brook, but as they drive out of town, Brook’s truck veers off the rickety Alliance River bridge. The truck falls into the icy river, killing both Joseph and his father. There is a funeral for Joseph, and the Hurds bury him alongside other deceased members of their family.

A year after Joseph’s death, the Hurds adopt Jupiter. Jack becomes her surrogate brother, supporting her as he supported her father Joseph.