Ethan Learner, portrayed by Joaquin Phoenix, along with his wife Grace, played by Jennifer Connelly, faces a heartbreaking tragedy following their 10-year-old son Josh’s musical recital. While en route home, a pit stop at a gas station on Reservation Road for their young daughter Emma leads to a devastating event. In a moment of sheer unexpected chaos, Josh is struck and killed by an SUV. The driver, Dwight Arno, characterized by Mark Ruffalo, briefly hesitates but, consumed by panic, flees the scene. The state police are summoned, initiating an investigation into the hit-and-run.
As time progresses, both Ethan and Dwight spiral into deep emotional distress over the incident. Dwight, a divorced attorney, is embroiled in a contentious relationship with his ex-wife Ruth regarding their son Lucas. On the night of the accident, he was driving Lucas home, pressed for time due to Ruth's multiple calls. Lucas, not wearing his seatbelt, suffers injuries in the crash, leading to a fierce confrontation between Dwight and Ruth, who threatens to revoke his visitation rights when she sees their son hurt.
Dwight wrestles with his conscience, longing to turn himself in yet fearing the prison sentence that awaits him. He conceals his battered SUV in his garage while the police begin their search based on a vague vehicle description. Meanwhile, the Learner family is shattered. With the police investigation appearing stagnant, Ethan takes matters into his own hands, joining an online support group for parents of hit-and-run victims and hiring an attorney, who he unknowingly discovers to be Dwight.
Grace struggles to hold the family together, slipping into a state of neglect, even disposing of Josh's toys to "move on." This decision stirs a furious confrontation with Ethan, who cannot comprehend her approach. As Dwight attempts to maintain his relationship with Lucas amidst his paranoia, he faces reprimands at work for failing to attend to his clients, especially Ethan.
As tension escalates, Ethan identifies a damaged SUV that resembles the one involved in his son’s death and unknowingly trespasses while taking photos, leading to his arrest. In a twist of fate, Dwight bails him out. At a school event honoring Josh, coincidentally hosted by Ruth, Ethan recognizes Dwight and connects the dots.
Determined to uncover the truth, Ethan discovers a photo of Dwight and Lucas with the SUV, igniting a dangerous quest for vengeance. Armed with newfound evidence and a gun, Ethan confronts Dwight, who is torn between confessing and pleading for his life. Just as Ethan grapples with his emotions, Lucas innocently disrupts their confrontation. In an unexpected turn, Ethan spares Dwight, tossing the gun aside, while Dwight discards it into a lake. The film concludes with Lucas watching a heartfelt video from Dwight, encapsulating the complicated emotions surrounding guilt, loss, and redemption.
