In 1961, the Indian Horse family — an Ojibway clan living in the wilderness of Northern Ontario near Redditt — consists of eight-year-old Forrest Goodluck as Saul, his grandmother Edna Manitowabi as Naomi, and his Christian parents John and Mary. They hide from authorities who had previously taken Saul's siblings to residential schools, clinging to their ancestral lands. When Skye Pelletier as Benjamin suddenly returns after escaping a residential school, the family relocates to Gods Lake, a remote place where their ancestors once lived. Benjamin soon dies of tuberculosis, and his parents take his body and do not return. That winter, Naomi and Saul attempt to reach Minaki, but their canoe overturns, supplies run dry, and Naomi dies. Saul is found by the authorities and taken to St. Jerome's Indian Residential School in White River.
At St. Jerome's, led by Michael Murphy as Father Quinney and Irene Poole as Sister Ignacia, Saul witnesses daily abuse of the children, some of whom die. In the same year, Michiel Huisman as Father Gaston Leboutilier joins the faculty and sets up a hockey team for the older boys. Inspired by Hockey Night in Canada, Saul begs to be let on the team despite his age, and is assigned to clean the rink; Saul uses this time to practice hockey, using assorted trash as a makeshift puck. When a player is injured during a scrimmage, Father Leboutilier lets Saul substitute and, astounded by his skill, allows him to join the team. Saul becomes St. Jerome's star player and is invited to play for White River's midget hockey team, though he is soon kicked out over his ethnicity.
In 1966, Saul, now thirteen, is invited by Fred Kelly to leave St. Jerome's and live with his Ojibway family in Manitouwadge to play for the Manitouwadge Moose junior hockey team, coached by Fred and captained by his son Will Strongheart as Virgil. He quickly bonds with Virgil and the Moose, leading them to victory against other reserve teams. Saul reunites with Father Leboutilier after a game in Pic River, the last time he ever sees him. After defeating the Kapuskasing Chiefs, a non-Indigenous Northern Hockey Association team, the Moose begin to travel to other towns along the Trans-Canada Highway. Yet they face heckling and violence over their ethnicity, culminating in an incident near Chapleau where a group of men beat and urinate on all of the Moose players except Saul, whom they spare for his age and skill. At sixteen, Saul is scouted by the Toronto Marlboros, a feeder team for the Toronto Maple Leafs; though hesitant, Virgil and the Moose persuade him to join the Marlboros.
In Toronto, Saul attends the Marlboros' training camp and makes the team, but he begins to react violently to the incessant discrimination he faces there, and leaves after being benched indefinitely. Saul returns to Manitouwadge and re-joins the Moose, but his aggressive behavior alienates his teammates. Realizing he is no longer welcome, Saul leaves town once he turns eighteen and becomes an alcoholic hobo. After roaming Canada for several years, in 1978 he briefly lives as a farmhand in Redditt but, feeling empty inside, leaves for Winnipeg, where he has a seizure and is hospitalized. Saul is accepted by the New Dawn Centre, an Indigenous rehabilitation centre, where he meets his counselor Moses and has a vivid spiritual experience in which he sees his deceased family. In 1986, Saul visits the now-shuttered St. Jerome's and breaks down in the abandoned hockey rink as he finally acknowledges his trauma: Father Leboutilier routinely molested and raped Saul, who used hockey as a means of escapism. Saul returns to Minaki and takes a boat to Gods Lake, where he has another spiritual experience in which he speaks with his great-grandfather Shabogeesick, the first "Indian Horse."
Saul travels to Manitouwadge, where he reconnects with Fred and his wife Martha, both residential school survivors who have endured abuse. He also reunites with Virgil, who is coaching his son Billy's bantam hockey team. Saul contemplates coaching the bantam team, and Virgil invites him to a reunion hockey game with the Moose. Later that night, while waiting for Virgil and the Moose, Saul discovers a ball of tape on the rink and begins practicing with it once more, mirroring the early days at St. Jerome's.
