Years after the events of Monsters University and Monsters, Inc., the monster world has changed completely. Laughter now fuels Monstropolis, and Mike and Sulley are celebrated heroes who helped create a better, more sustainable energy system. However, beneath all the laughter, Sulley still wonders about Boo—the little girl who changed everything. Her door was shredded years ago, but the question of what happened to her has never left his mind. The film begins when a new discovery at Monsters, Inc. hints that Boo’s door might still exist somewhere in the company’s archives.

Curiosity drives Sulley and Mike to investigate, and they stumble upon a secret storage room filled with forgotten doors from the old scream-processing era. Among them, one door glows faintly with an energy that feels familiar. When they activate it, Sulley’s heart skips a beat—Boo’s laughter echoes faintly through the machinery. Against the company’s regulations, Sulley decides to go through the door one last time. To his surprise, he finds himself back in the human world, years after he last saw Boo.

Boo, now a teenager, still remembers her “kitty,” though she has often wondered if he was real or just a dream from her childhood. When she sees Sulley again, her shock turns into joy, but also confusion—how could her childhood monster still exist? The emotional reunion between Boo and Sulley becomes the heart of the story. However, their connection threatens to expose the monster world to humans, something that could endanger both sides.

As Mike and Sulley struggle to fix the situation, a new threat emerges. A former Monsters, Inc. technician discovers the door technology can be weaponized, allowing monsters to cross into the human world freely. The balance between laughter and fear is once again at risk. Together with Boo, they must protect both worlds by repairing the door system before it collapses entirely.

In the climax, Boo helps the monsters understand that humans no longer need to fear them. Her belief in friendship and kindness bridges the two worlds in a way laughter alone never could. The final moments show Sulley and Boo saying goodbye once more, but this time with hope rather than sadness.

Monsters, Inc. 3: Boo’s Door beautifully closes the trilogy with a mix of nostalgia, humor, and emotion. It reminds audiences that growing up doesn’t mean forgetting wonder—and that even the scariest monsters can have the biggest hearts.