Michael "Mike" Wazowski, a six-year-old monster, has the thrilling opportunity to visit Monsters Inc., a prominent scaring company, during a school outing. There, the class encounters Frank McCay, one of the scarers who enter the human world to frighten children at night, harnessing their screams as an energy source to power the monster realm. Captivated by this idea, Mike sneaks through a door just in time to witness Frank’s impressive scare, eventually returning with him to the monster world. Although Frank chides Mike for his sneaky antics, he can't help but admire Mike’s stealth and gifts him his Monsters University hat as a cherished keepsake. Lost in dreams of becoming a scarer, Mike is undeterred by the scolding from his teacher.
Fast forward roughly eleven years, and Mike is now a scare major at Monsters University. On his initial day there, he encounters his quirky roommate, Randall "Randy" Boggs, a monster with the unique ability to turn invisible. During their first scare class, Mike attempts to answer a question but is interrupted by the arrogant, hefty blue monster, James P. "Sulley" Sullivan. The strict Dean of the scare program, Abigail Hardscrabble, lays down the law, emphasizing that they must nail the final exam to remain in the program.
One night while poring over his studies, Mike finds himself in an unexpected clash with Sulley, who barges in trying to hide a stolen Fear Tech pig mascot. Their quarrel leads to a chaotic chase after the mischievous pig, culminating in Sulley claiming credit for capturing it. This earns him an invitation to join Roar Omega Roar, the esteemed fraternity on campus—leaving Mike feeling snubbed.
As Mike diligently works to excel in class and consistently provides correct answers, Sulley relies solely on his natural scaring prowess, ultimately leading to his downfall. When the final exam fails to meet expectations due to their rivalry, both are dropped from the program, prompting Roar Omega Roar to expel Sulley as well. Disillusioned with his lackluster major, Mike resolves to prove his worth by entering the Scare Games, a competition requiring fraternity or sorority membership. To bolster their team, Sulley unexpectedly offers his expertise, and Mike, though hesitant, agrees.
Mike strikes a deal with Dean Hardscrabble, who remains skeptical, vowing that if they win, they can all rejoin the scaring program—while losing means Mike has to leave Monsters University. Despite Sulley's expectation to carry the team, Mike believes that rigorous training will help them thrive.
Oozma Kappa, the misfit fraternity, stumbles through the early rounds of the Games but astonishingly advances to the next stage when another team gets disqualified. They then find themselves at a Roar Omega Roar party, where initial acceptance quickly turns into ridicule. Trying to lift their spirits, Mike organizes a secret excursion to Monsters, Inc., which motivates Oozma Kappa to harness their creativity and skill, propelling them to the finals against Roar Omega Roar.
Even as they reach the final round, Sulley harbors doubts over Mike's potential due to his perceived lack of natural talent. Following their triumph, Mike learns Sulley tampered with the results to boost his score. Heartbroken and desperate to prove himself, he enters a door to the human world, only to discover that it leads to a summer camp full of oblivious children, failing to scare them.
Back at the university, Sulley confesses to Dean Hardscrabble about his cheating, only to discover that Mike has broken into the door lab. Realizing the urgency of the situation, Sulley rushes through the door in search of Mike. After their reconciliation, the duo finds themselves pursued by human adults; soon, they realize they must generate enough scream energy to power the door back to their world. Working together, they scare the adults effectively, allowing them to return to the lab.
Their actions result in expulsion from the university, but Hardscrabble is impressed by Oozma Kappa's performance, leading to their acceptance into the scaring program the following semester. As they bid farewell, Hardscrabble acknowledges that Mike and Sulley were the first to truly surprise her and wishes them success in their future endeavors.
Ultimately, Mike and Sulley begin their new chapter at Monsters, Inc., taking on positions in the company mailroom under the supervision of the Abominable Snowman. As they navigate their way through the company, they eventually ascend to become key members of the Scarer Team, fulfilling their lifelong dreams.
