Casey, played by Jesse Eisenberg, is a timid and unassertive accountant who is attacked by motorcycle riders while walking home. Following the incident, Casey experiences anxiety and fear, leading him to a self-defense class. The class is taught by Sensei, played by Alessandro Nivola, who is revered by his students and for his stoic masculinity.
As Casey becomes captivated by the teachings of Sensei and the hypermasculine culture of the class, he begins to unravel the sinister world of the self-defense fraternity, one that is fueled by violence, unspeakable rituals, and sexist attitudes. Sensei's teachings slowly take control of Casey's life, as he begins to assert himself more in his work and personal life.
One of the few prominent female students, Anna, played by Imogen Poots, becomes a source of inspiration for Casey as he witnesses how she resists the sexism promoted by Sensei and other male students. Anna challenges the notion that being hypermasculine and dominating is the only path to power and mastery.
As Casey becomes more deeply entrenched within Sensei's dark world, he begins to question the destructive and misogynistic beliefs underpinning his teachings, leading to a shocking climax that tests his newfound sense of empowerment. The movie explores the themes of toxic masculinity and the perversion of power in a biting and satirical manner.