Hank Mitchell, a bookkeeper, and his pregnant wife, Sarah, reside in the tranquil countryside of Wright County, Minnesota. While Hank holds the distinction of being one of the town's few college graduates, Sarah serves as the local librarian. One day, while chasing a fox into the woods alongside his slow-witted older brother, Jacob, and their friend Lou Chambers, they unexpectedly stumble upon a downed airplane. Curiosity piqued, Hank investigates the interior, only to discover a lifeless body and a bag brimming with $4.4 million in $100 bills. Although he proposes turning the found fortune over to the authorities, Jacob and Lou persuade him to keep it a secret. Hank ultimately decides to stash the money in his home until spring thaw when they'll decide what to do if the plane remains undiscovered.
As chance would have it, Sheriff Carl Jenkins passes by and engages in a friendly chat. Hank adheres to their agreed-upon story, but Jacob's slip about the plane raises some suspicion. Once the sheriff departs, the trio swears to keep their secret, but Hank can't help but confide in Sarah almost immediately.
Sarah, worried about the consequences, proposes returning a fraction of the money to the aircraft site to thwart suspicion. Hank ends up taking Jacob with him, but their encounter with elderly farmer Dwight Stephanson takes a dark turn. In a panic that their secret may be exposed, Jacob strikes Dwight, and as they struggle, Hank suffocates him. They dispose of Dwight's body off a bridge, making it appear as an accident. Meanwhile, Sarah’s investigations reveal that the money they found is linked to a ransom for a kidnapped heiress taken by two brothers.
The tension rises that same night when Lou, inebriated, pressures Hank for his share of the money. Upon learning about Dwight's murder from Jacob, Lou threatens to go to the police if Hank doesn’t pay up. After giving birth to their daughter, Sarah schemes further, convincing Hank to entrap Lou by getting him drunk, leading him into a false confession about Dwight's murder which Hank records. The ruse works well, leaving Jacob feeling conflicted over betraying his friend. The situation escalates when Lou, realizing he has been tricked, confronts Hank at gunpoint. In an attempt to protect Hank, Jacob ends up shooting Lou, which triggers a violent cascade that leads to Hank killing Lou’s wife, Nancy, in self-defense as she retaliates.
To cover their tracks, Hank and Jacob present the entire tragedy as a domestic dispute gone wrong. When Sheriff Carl requests their assistance in searching for the missing plane, Sarah grows increasingly suspicious of the lawmen involved. She soon discovers that the FBI agent, Baxter, isn't what he seems and alerts Hank, who prepares for the worst by taking a service revolver. As the group combs the woods, Hank's instincts to protect himself and his secrets lead to a violent confrontation. When Carl finds the plane, Hank tries to warn him of Baxter’s true intentions, but it’s too late. Hank manages to turn the situation around, killing Baxter in self-defense.
Now faced with a web of deceit and violence, Hank's world begins to crumble. Jacob implores Hank to end his life and frame it all as Baxter’s doing, but Hank, caught in a moral battle, refuses. In a heartbreaking twist, Jacob chooses his own fate, and Hank, feeling trapped, is forced to take his brother’s life. The aftermath leaves Hank seemingly exonerated by legitimate FBI agents, who reveal they had been tracking the money due to serial numbers all along.
Now realizing he cannot safely use any of the remaining cash, Hank decides to destroy it. When Sarah fights against this decision, reminding him of the life of poverty that awaits them, Hank resolutely pushes her aside. In his closing thoughts, Hank reflects on how the choices they made have woven a permanent nightmare into their existence, with no escape from the haunting memories of their own making.
