Casey Davies, portrayed by Jesse Eisenberg, is an accountant who struggles to find his place in the unwelcoming atmosphere of his workplace. After a harrowing attack from a group he encounters one night at the supermarket, Casey takes time off to heal, but his feelings of insecurity intensify. Considering a purchase of a firearm for his protection, he stumbles upon a karate dojo run by the enigmatic Sensei. Intrigued by a free trial class, Casey decides to immerse himself in the dojo life rather than succumb to fear.
In the dojo, he meets Anna, a brown belt and the sole female instructor for children, who becomes a pivotal figure in his journey. He also forms a friendship with Henry, a blue belt student. As he begins to train, Casey impresses both his classmates and Sensei, earning a promotion to yellow belt. However, Henry and Anna are overlooked for their subsequent promotions, creating an atmosphere of unspoken tension.
When invited to the dojo's more intense night classes, Casey witnesses firsthand the brutal side of martial arts. Henry, determined to prove himself, faces a violent reprimand from Sensei, suffering a broken elbow during a demonstration. Meanwhile, Anna refuses to be marginalized, delivering a crushing defeat to the newly promoted black belt, Thomas, in a bid to prove she deserves a higher rank despite Sensei’s dismissive comments about her gender.
Under Sensei's guidance, Casey adopts a new persona, becoming more assertive and aggressive. He returns to his accounting job, surprising his colleagues with his newfound bravado, and even resorts to violence against his boss, resulting in his eventual termination. Sensei then offers him a position to manage the dojo's finances, further entrenching Casey in this newfound lifestyle. During a candid moment, Casey confides in Sensei about the trauma from his recent attack, hoping for some form of retribution.
The situation takes a dark turn when Sensei informs Casey that he has spotted one of the attackers at a bar, urging him to take action. In a moment of fury, Casey severely injures the man only to discover, in a shocking twist, that he was recording the incident, having mistakenly harmed an innocent individual.
Distraught by recent events, Casey returns home only to find his beloved dog unconscious. After a grim visit to the vet, he learns it has died, a demise reminiscent of a technique he learned in his first class. Furious, he confronts Sensei, who denies any culpability but implies that Casey's prior violent actions are being monitored.
Things spiral out of control when Casey and Anna find themselves embroiled in a scheme to attack unsuspecting individuals, only for them to discover one of their targets is an undercover police officer. In the ensuing chaos, Anna is shot, and in a moment of blind rage, Casey takes a life, all under Sensei's approving gaze.
Determined to uncover the dojo's sinister secrets, Casey finds himself in a restricted area where chilling evidence awaits: a crematorium, a corpse hanging by a blue belt, and video tapes that reveal Sensei and some classmates were behind the very gang that had attacked him. It becomes clear that Anna had protected him during that initial assault.
Confronting Sensei the next day, Casey challenges him to a fateful duel. Equipped for combat, he takes drastic measures, ultimately ending Sensei's reign with a gunshot—a shocking finale to their bitter rivalry.
Now the de facto leader of the dojo, Casey fabricates a story of triumph over Sensei, asserting his dominance and exacting revenge on those who wronged him. Recognizing Anna's strength, he promotes her to black belt, enabling her to teach a new philosophy of combat—one that champions compassion over savagery. Now, Casey takes on the responsibility of guiding the next generation, finally finding his purpose as a teacher in a place he once sought refuge.
