A man is attacked at random on the street. He enlists at a local dojo, led by a charismatic and mysterious sensei, in an effort to learn how to defend himself.
Casey Davies is a meek and timid accountant who suffers a mugging by a motorcycle gang. In an attempt to toughen himself up, he joins a karate dojo led by a mysterious Sensei. He also meets a woman named Anna, who is the only female member of the dojo who has been trying to prove her worth since the dojo first opened.
While spending more time under Sensei’s tutelage, Casey becomes more assertive but in a toxic manner. He also finds that Sensei is having him do insane tasks like attacking a man who is allegedly one of the muggers (which turns out to be false), and then attacking other people on motorcycles (indicating the students were the ones who mugged him). Casey later finds proof of Sensei’s dirty deeds via paperwork and videos that confirm he led the mugging on Casey.
Casey challenges Sensei to a duel to the death, but instead just shoots him in the head. He fools the other students into thinking that he beat Sensei using a fabled technique known only by the dojo’s late grandmaster, and he then trains his new German shepherd to kill Thomas, a student that earlier killed Casey’s little dachsund. Casey then gives Anna the black belt she had long coveted, and she is promoted to sensei with the intent of teaching a more compassionate karate style, while Casey teaches the children.
Casey Davies (Jesse Eisenberg) is a mild-mannered accountant who does not seem to fit in with his work environment. While getting food for his pet Dachshund late one evening, Casey is confronted by a motorcycle gang, who ask if he owns a gun before brutally attacking him. While given time off from work to recuperate, he grows insecure and strongly considers purchasing a firearm for protection. But when he comes across a karate dojo, presided by “Sensei” (Alessandro Nivola), Casey takes advantage of a free trial class, opting to take further day classes rather than purchase a firearm. Casey meets Anna (Imogen Poots), a brown belt and the only female student of the dojo who teaches the children’s classes, and befriends Henry (David Zellner), a blue belt student. With time, Casey begins to perform well enough to catch the attention of his classmates and Sensei. He earns a promotion to a yellow belt, but both Anna and Henry are snubbed for their next ranks.
Sensei invites Casey to the dojo’s night classes, which are more extreme and brutal than the regular classes. Henry joins the class uninvited, but when he is called upon to help demonstrate, Sensei breaks his elbow at the joint and expels him from the dojo. During a sparring session, Anna brutally beats Thomas (Steve Terada), the newly promoted black belt student, aiming to prove her skill to Sensei despite her gender, which Sensei claims disqualifies her from earning a black belt.
After a two month absence from his accounting job, Casey gets terminated but is then offered a temporary position by Sensei to be the dojo’s accountant. Casey reveals to Sensei that he is taking classes in response to the traumatic experience of being attacked by the motorcycle gang. Days later, Sensei calls Casey telling him that he has spotted one of the gang members at a bar, encouraging him to attack the drunken suspect. Casey gravely injures him, but is surprised to find Sensei recording the confrontation, realizing that the suspect was innocent.
A distraught Casey returns home to find his dog unconscious. He is later pronounced dead at the hospital, due to trauma that resembles either a punch or a kick. Casey returns to the dojo and accuses Sensei of killing his dog, which he denies. During the evening’s session, the entire class goes out to ride motorcycles — which look familiar to Casey — and act as vigilantes. As partners, Anna and Casey target an undercover police officer. Anna is shot in the leg, but Casey kills the cop to Sensei’s approval. Casey takes Anna home and finds a German Shepherd at his house, given by Sensei. Casey returns to the dojo to investigate further.
In a restricted section of the dojo, Casey finds several disturbing things: A working crematorium; Henry’s corpse, hanging by his own blue belt; a hit-list; and a series of video tapes that reveal Sensei and some of his own classmates, including Anna and Thomas, as the motorcycle gang that attacked him. When Sensei returns in the morning, Casey challenges him to a fight to the death, as the dojo’s late grandmaster is known for. Suited up, the two bow, and Casey pulls out a gun from his gi and shoots Sensei in the head, killing him.
At the evening class, Casey tells the other students that he killed Sensei in a duel by punching his pointer finger through Sensei’s skull, having previously dipped his finger into the bullet wound as proof. Casey takes charge of the class and sics his new dog on Thomas, whom he discovers was responsible for killing his dachshund. Casey then promotes Anna to a full black belt and she becomes the new sensei; teaching her beliefs of compassionate combat over ruthless aggression. Casey is then promoted to teaching the children’s classes in her stead.
The film starts in a diner where Casey Davies (Jesse Eisenberg) is eating alone. A French couple enters and sits down not far from him. In their own language, they play a “game” where they imagine what kind of person Casey is. He gets in his car and plays a tape for learning to speak French, and the phrase he learns is one pleading in French to not be hurt because he is a tourist.
Casey works as an accountant. He tries to talk to three coworkers in the break room, but he comes off as awkward. When the guys learn that Casey audits their expenses, they tell him to get lost. He also lives at home with nobody but his little dachsund. He sees that the dog is out of food. On his way to the pet store to buy food, he is approached by two men on a motorcycle, with one of them asking if he has a gun. Casey later walks home with the food when the men on the motorcycle, plus a few other assailants, start chasing him before they catch and beat him unconscious.
Casey wakes up in the hospital after his mugging and is told by his boss Grant (Hauke Bahr) that he can take as much time off as he needs. Casey becomes fearful for his life, and after watching a gangster movie, he feels compelled to buy a gun. He goes to a gun store and tries to ask for a handgun, and is told that he must go through the background check and waiting period before he can actually get it.
While walking around town, Casey passes by a karate dojo. He enters and watches as the Sensei (Alessandro Nivola) teaches his class. He appears to incorporate karate into every facet of his life. After the lesson, Casey talks to Sensei and asks if he can join. Sensei invites him to the next day’s class for a free trial to see for himself. Casey also meets Anna (Imogen Poots), the only female member of the dojo who is a brown belt and also gives classes to kids. She demonstrates a sleeper hold on a child before talking to Casey, where she immediately chastises him for wearing shoes on the mat.
Casey begins his lessons, starting as a white belt. He meets Henry (David Zellner), who is friendly to him and is also working his way to a promotion in color ranks. During Casey’s first lessons, he is taught how to punch with his feet. He spars with another student, Kennith (Phillip Andre Botello), who knocks Casey off his feet on the first try. Casey also does manage to get a good “punch-kick” here and there. Sensei also tells the students about the dojo’s late grandmaster, who was killed after being shot, and this is why Sensei believes that people who use guns are weak. He also mentions a move that only the grandmaster knew wherein he could kill a man by pushing his index finger through his skull.
One afternoon while buying groceries, Casey goes back to his car to see some jerk in a pick-up truck door-bang his car. When Casey confronts the man about it, the man intimidates him and knocks the groceries out of his bag. Casey ends up driving home crying.
During lessons, Henry tells Casey that he thinks he is getting promoted to black belt. Sensei does a promotion ceremony where he upgrades Casey to a yellow belt. When Henry brings up the other promotion, Sensei says he is promoting someone to black belt, but it is another member, Thomas (Steve Terada). Later on, Sensei notices Casey performing less actively than before. They talk in Sensei’s office, where Casey brings up the mugging and how he is terrified of men larger than him, and that he wants to become that which intimidates him. Sensei invites Casey to check out the night classes.
Casey goes to the night classes and finds Henry there, even though he was not invited, and he also thinks Casey betrayed him by showing up without telling him. Sensei has Henry go up for a demonstration, and Sensei brutally snaps Henry’s arm at the elbow. In another instance, Anna spars with Thomas and puts him in a headlock. While unconscious, she proceeds to smash his face in with her fists as retribution for him getting a black belt before her. Sensei then disqualifies her from getting a black belt. He later has Casey cool off in the maintenance room, which is also Anna’s changing room. She gives him a massage to cool off, but refuses to talk to him about why she attacked Thomas.
Casey gets a message from Grant saying that he needs to come back to work since it has been two months. He starts to utilize an assertive demeanor, which really makes him come off as a dick. He punches Grant in the throat after he invites Casey to have dinner with him and his wife, and then he talks to the guys in the break room and insults them before appearing to get them on his side. Naturally, he is fired, but Sensei offers him a position as an accountant for the dojo.
Casey starts to curry favor with Sensei and the other students (save for Anna) by applying himself more, as well as getting everyone regular belts that match their karate colors.
Sensei later calls Casey to tell him that he has spotted one of the bikers that attacked him. He meets Sensei at a bar and is ordered to go confront him. Casey follows the man, who is very drunk, to a parking garage where he appears to be going to a motorcycle. Casey ends up hitting the man in the stomach, causing him to fall and hit his head on the pavement, causing it to bleed. Sensei then comes out and starts recording the incident. Casey then takes the man’s keys and sees that he was going to a regular bicycle, and he runs away in horror. When he gets home, he finds his dachsund on the floor unconscious. He takes him to the hospital, but the doctor tells Casey that the dog died from what appears to be a kick to his ribs. However, there was blood found on his teeth, meaning he bit whoever attacked him.
Casey goes back to the dojo and deliberately breaks all the rules (eating, drinking, and wearing shoes on the mat) to piss of Sensei after accusing him of killing his dog. Sensei comes out and denies the accusations before knocking Casey on his back. Casey later goes home to see that Sensei left him a German shepherd with a note to remind Casey that he must establish himself as the alpha, or the dog will rip his throat out.
Casey goes for another night class, where Sensei brings him, Anna, Thomas, and Kennith out for a motorcycle run. Casey rides with Anna, who approaches an undercover cop in the same way that Casey was approached earlier. The cop shoots Anna in the leg, and before anything else goes wrong, Casey uses his moves to attack the cop, but he ends up killing him by slamming his head against a fire hydrant. Sensei chastises Anna for not asking the man if he has a gun (just like the ones that mugged Casey). Sensei tells Casey that he has earned a red stripe on his belt for taking a life, and he notes that Anna has one too. Casey takes Anna home, and she tells him that early on when she started out in the dojo, she held back because they were never going to take her seriously as a woman. The champion at the time attacked her in the dressing room, and she strangled him to death with his own belt. She thanks Casey for taking her home but tells him not to return to the dojo.
Now having grown more concerned, Casey goes back to the dojo after hours to do some more digging. He discovers that the dojo has a crematorium, and also finds Henry’s body, having been hanged by his own belt. There are also tapes, including one of Casey’s mugging. It confirms that Sensei, Anna, and Thomas were the ones who attacked him, with Thomas being the one who knocked him out. It was part of some kind of ploy for Sensei to get others like him to turn their lives around. After leaving, Casey sees the pick-up truck from the jerk outside the grocery store, and he chops the left mirror off and leaves a dent in it. Later on, we see Sensei find Henry’s body and throw it in the crematorium.
Casey approaches Sensei the next day and tells him flat-out that he is going to kill him. He also calls him by his real first name, Leslie, just to provoke him, especially since he earlier mocked Casey for having a feminine-sounding name. He also brings up how Sensei is blackmailing other former students into paying membership fees long after they leave. Casey challenges him to a duel to the death, which Sensei is confident he will win. The two suit up and bow, but Casey pulls a gun out of his gi and puts a bullet in Sensei’s head. He then delivers two specific quotes that he wanted to say after killing him, and he then sticks his finger in the bullet wound.
During the night class, Casey takes over and leaves Sensei’s body on the floor for others to see. He tells the students that he killed Sensei with the finger-punch technique that the grandmaster supposedly knew. Before he continues, he tells the students to hold out their hands. He sees bite marks on Thomas’s wrist, confirming that it was he who killed Casey’s dachsund. He brings the German shepherd out, now having trained him and learned to command him in German. The dog proceeds to rip out Thomas’s throat. Casey then gives Anna the black belt and puts her in charge of the dojo, where she promises to display a more compassionate form of fighting. Meanwhile, Casey goes on to teach the kids.