The film begins in a Jiu-Jitsu academy, where Mike Terry (Chiwetel Ejiofor) instructs an advanced class using an unconventional training method. In his unique style, each participant draws one of three marbles—two black and one white—before sparring. Those who draw black face a handicap set by a spin of a wheel. Joe (Max Martini), a police officer vying for a black belt, draws a black marble and must fight with his hands tied. Despite showing impressive skill, Joe ultimately fails to escape a chokehold, prompting Mike to console him by pointing out that he had the ability to escape if he had been more focused.
Meanwhile, Laura (Emily Mortimer) is driving in torrential rain, urgently seeking a pharmacy to fill a prescription. Distracted by her phone, she accidentally sideswipes a parked truck. In her search for the truck's owner, she stumbles into the Jiu-Jitsu studio, where Mike is illustrating to Joe how his police-issued holster can be a liability. Observing Laura's distress, Joe offers to help her with her coat, but in a moment of shock, she inadvertently grabs his sidearm, causing it to discharge and shatter the academy's front window. Collecting the shell casing afterward, Laura leaves Mike and Joe astonished at how quickly fear took control of her.
In an effort to suppress the incident, Mike and Joe agree to conceal the truth, worrying that the police could charge Laura with attempted murder despite her obvious distress. They decide to manipulate the situation into an “act of God” to avoid insurance repercussions, leading to tension between Mike and his wife, Sondra (Alice Braga), regarding their financial struggles as the academy faces bankruptcy. Sondra supports Mike by advising him to seek a loan from her brother, Ricardo (John Machado), a mixed martial arts champion.
When Mike approaches Ricardo at his club, he discovers the facility lacks proper security, a situation that Joe was originally hired to manage. Ricardo is unavailable, so Mike speaks with Bruno (Rodrigo Santoro), Sondra’s other brother. During this visit, Mike witnesses a talented magician showcasing absurd sleight of hand, piquing his interest. In the club's back office, Bruno and fight promoter Marty Brown (Ricky Jay) discuss plans for an upcoming championship match involving Ricardo. Marty hints at possibly rigging the fight as Bruno expresses skepticism about his brother's willingness to compromise principles.
Later, while out, Chet Frank (Tim Allen), a fading Hollywood action star, enters the establishment and awkwardly gets embroiled in a confrontation, which Mike steps in to break up. The next day, Chet sends Mike a lavish watch and an invitation to dinner as thanks for his intervention. Mike, wanting to help Joe, gifts the watch to him, urging Joe to pawn it instead.
At the academy, Mike finds that Laura has returned to take responsibility for fixing the broken window and to enroll in his class. During a dinner with Chet's wife, business prospects arise as she expresses interest in Sondra's fashion creations. After hearing about Mike's training methods, Chet proposes that Mike join him on set, leading to a promising opportunity for collaboration.
As Mike trains Laura, she reveals her traumatic past involving a violent assault, which explains her jumpiness. In a moment of tension, Mike demonstrates how she could have defended herself, leading to an emotional breakthrough. Meanwhile, Joe faces consequences at work over the pawned watch and learns it was stolen, causing a rift between him and Mike.
The plot thickens when Mike discovers that Sondra’s business venture is fraught with risks due to disconnected contact information for Chet's associates, leaving them in a financially precarious position. As the fighting promotions escalate, Mike's training techniques are appropriated by the promoters for advertising, igniting his sense of betrayal. Laura becomes essential in pursuing legal action, but their attempts to prove theft falter when they are met with serious threats that implicate both her and Mike in a cover-up.
Tragedy strikes when Joe, consumed by guilt over the mess, takes his own life, leaving behind a devastated wife and unanswered questions. Fueled by a sense of responsibility, Mike ultimately decides to fight in the upcoming event, abandoning his strict code of honor for financial survival.
Upon arriving at the fight, Mike learns about the manipulative theatrics surrounding his training philosophy, which has been twisted into a spectacle for profit. After uncovering that the match is fixed, Mike confronts the promoters, who shamelessly dismiss Joe's suicide as collateral damage. After a heated exchange, Mike's character is tested as he faces off against Ricardo in a dramatic showdown, eventually defeating him amidst widespread attention.
As the audience witnesses his triumph, Mike receives an emblematic ivory-studded belt from Morisaki (Enson Inoue) and earns the coveted Redbelt from The Professor, establishing himself as a formidable fighter and true disciple of the craft.
