Former collegiate wrestler Scott Voss finds himself disillusioned and unfulfilled as a 42-year-old biology teacher at the struggling Wilkinson High School. In his role, he's often tardy to class, leading Principal Duke Betcher to cut his vacation pay. Scott discourages his students from delving deep into the subject or even asking questions, leaving them uninspired.
As budget cuts threaten the school's music program and the job of its devoted teacher, Marty Streb, who's eagerly awaiting a baby with his wife, Scott becomes increasingly aware of the urgency of the situation. Despite Marty's passion for teaching, Principal Betcher prioritizes the football team, which accounts for a significant portion of the school's funding.
Determined to save his fellow teacher's job and help the students, Scott embarks on a quest to raise $48,000 by the end of the school year. Initially, he checks with his brother Eric for extra work, only to find that job prospects are bleak, particularly with Eric's wife Lauren managing their finances strictly. Scott starts by teaching a night citizenship class but soon accepts outside tutoring for one of his students, Niko, who is facing deportation back to Holland if he fails his test.
Upon visiting Niko's apartment, Scott discovers that Niko is a former mixed martial arts fighter. While watching the UFC, inspiration strikes Scott when he realizes that fighters can earn significant payouts. Motivated, Scott decides to enter the MMA ring himself. Although Niko doubts his age and experience, he eventually agrees to train Scott when he sees his resolve.
Encouraged by Niko and Marty, Scott begins with small unsanctioned bouts, racking up $750 for each fight. He selects "Here Comes the Boom" as his entrance music, although he is forced to revert to Marty's intimidation tactics later on. With each victory—starting with a knockout of opponent "Lucky" Patrick Murray—Scott realizes he can expedite his fundraising efforts. As he dedicates himself to training, he garners admiration at school for his attempts to save Marty's job.
Meanwhile, one of Scott's students, Malia, creatively helps Niko prepare for his citizenship test by transforming the study material into memorable songs. As Scott’s relationship with school nurse Bella blossoms, he reignites his passion for teaching and his sense of purpose. During his training, Mark informs Scott of an upcoming UFC fight, with a guaranteed payout even in defeat, which initially receives pushback from Niko due to his previous spinal injury that ended his own fighting career. Following a heart-to-heart, Scott and Niko agree to accept the fight.
The journey takes them to the MGM Grand in Las Vegas, where Scott learns that the funds he has worked so hard to secure for the music program have been embezzled, leaving him desperate to win and raise the necessary funds. Drawing strength from the support of his school, Scott steps into the octagon against Ken Dietrich, who underestimates him. Inspired by his students, Scott rallies, ultimately achieving victory and earning $50,000, along with newfound respect from Dietrich and a heartfelt kiss with Bella.
Amidst this narrative, a subplot emerges involving student Malia and her father, who initially wants her to abandon music to help with their struggling family restaurant. However, with encouragement from Scott and the addition of Eric as the restaurant's chef, Malia's father begins to appreciate her passions. The culmination of the film sees Malia sing at the UFC event, impressing her father and affirming her musical aspirations.
In a touching conclusion, Scott’s hard work pays off as Niko, along with all the students in his citizenship class, receives American citizenship at a ceremony, showcasing the profound impact of community support and perseverance.
