Walter "Lion" Kaminski, Jack O'Connell, is a gifted former professional boxer who drifts between a sewing factory job and shadowy, late-night underground bouts. His brother Stanley Kaminski, Charlie Hunnam, an ex-con and Lion's idealistic manager, keeps a delicate balance between loyalty and pressure as they squat in a dilapidated Fall River row house with their loyal whippet Ash. The bond between the two is strong, but their world is built on hard knocks, hunger, and the constant lure of the ring as a path out.
Stanley is deep in debt to Pepper, Jonathan Majors, a local gangster who runs a tight grip on the brothers. To settle the debt, Pepper proposes a dangerous plan: drive a young girl named Sky to Reno, Nevada, before Lion fights in an underground prizefight called Jungleland in San Francisco’s Chinatown. Pepper attends the fight and Lion realizes with a sting that Stanley bet on him to lose; enraged, Lion throws the match. Pepper, recognizing Lion’s raw talent despite the betrayal, offers a grim pathway to pay off the debt: complete the mission with Sky and then Lion will step into Jungleland. The terms are brutal, and the stakes—and the moral fog—grow heavier by the mile.
Sky, Jessica Barden, a streetwise and troubled young woman, befriends Lion at a bar along the way. Stanley, meanwhile, indulges in a one-night stand with a woman in their hotel, underscoring the tangled, morally gray currents running through the trio. Sky stages a distraction and drugs Lion with Xanax to steal the car keys before escaping. She crashes the SUV, and Stanley, upon catching up, locks Sky in their hotel room and calls Pepper for answers. It’s then revealed that Sky is an underage prostitute who must be delivered to Yates, a pimp who has past dealings with Stanley. Sky pleads with the brothers not to go to Reno, but the threat of Pepper’s violence—and the pressure of their own precarious situation—pushes them forward. Sky’s real name is Mary McGinty, and her estranged religious family back in Gary, Indiana, looms large in the tension of what awaits her. Yet the trio’s attempt to pull money from Sky’s guardians proves fruitless, and they drift toward a temporary home in a shuttered school, where Sky’s questions deepen and Lion’s concern for her grows.
The trio then makes a risky deal with the local garage: Lion can drive away with the car if he defeats two mechanics in a street-fight challenge. Lion rises to the test, dispatching both opponents with quiet power, and the road trip accelerates toward the West. A stop at a restaurant spirals into another argument as Lion accuses Stanley of manipulating him, while Stanley reveals the toll years of fighting have taken on his body—arthritis in his hands and memory troubles that cloud his judgment. Their tension peaks as Ash’s nonstop barking and a hostile note from a diner lead to them being forced out, and Stanley’s bitterness erupts when he secretly sells Ash to a father and son, a betrayal that triggers a brutal confrontation between brothers. They board a bus toward Reno, but Sky and Lion slip off in Carson City, leaving a sleeping Stanley behind.
Stanley is later abducted by Yates’ men for failing to deliver Sky. In a moment of desperation, Lion pleads with Sky to run away with him, but she refuses, and the two share a charged moment in a bathroom, where Lion learns Sky is pregnant with Yates’ child. Sky departs, leaving Stanley’s money and a little protection behind, money that Stanley uses to press on toward Yates’ hideout in Reno. Lion’s courage moves him to a confrontation with Yates’ henchmen, and he incapacitates one before killing Yates himself. A freed Stanley acknowledges that they could run away, but Lion’s focus remains on Jungleland. Stanley, overwhelmed by regret, breaks down and asks for forgiveness, which Lion grants with unspoken grace. Sky returns to the picture, bearing a ring robe Sky herself crafted as a parting gift, a symbol of care amid the chaos.
Together, they move toward Jungleland. Lion steps into the ring with everything he has, a performance that blends skill with raw emotion. He fights bravely and wins, but the victory is tempered by the arrival of the law; the police sweep the venue, searching for Stanley, who has left his ID behind at the Reno crime scene. Stanley makes the tough choice to turn himself in, allowing Lion to keep the prize money and the chance at a real life beyond the fight’s glare. In the final moment, Sky reappears at the venue, and the two share a quiet, newly tempered reunion amid the glow of Lion’s hard-won triumph.
