When a new, seasoned substitute teacher, Mrs. Brown, replaces a teacher who got stabbed by a student, she soon realizes that she faces the most hostile class in her entire teaching history. With her longstanding experience and no-nonsense attitude, she tries to convey strength and confidence in order to get the upper hand, but it doesn't take long until Santiago (16), a gangbanger wannabe, challenges her in a physical confrontation. During the scuffle, Santiago pushes Mrs. Brown to the ground and ends up urinating on her. This incident makes it clear; the class is completely out of control and no more teachers can be found.
After an emergency staff meeting, the Principal learns about some unconventional, underground substitute teacher who apparently has the power to convert rebellious students into future high school graduates, and all this in just one day. Having run out of options, the Principal reluctantly signs a confidentiality agreement and hires this mysterious teacher.
A few days later, Mister X, a former mercenary and epitome of a man, shows up at the school and kicks in the classroom door. The unruly youngsters are baffled as their new teacher uses a drill to install a heavy padlock onto the door; nobody leaves! The first conflict quickly escalates when Santiago refuses to turn off his ghetto blaster. After demolishing the radio, winning a physical fight, taking a bullet, and choking out a student, Mr. X's first lesson is established; RESPECT! Mr. X continues to employ a variety of politically incorrect tactics, teaching his students about life, values, purpose, teamwork, and most of all, about themselves. At the end of the day, the rebellious students have not only developed a new appreciation for life and each other, but also believe, for the first time, that graduating is indeed within their reach.