Monstropolis

Monstropolis is a world in Kingdom Hearts III. It is based on the Disney/Pixar film Monsters, Inc.. It is a city inhabited by monsters and powered by laughter energy provided by Monsters, Incorporated.

Development
Tetsuya Nomura revealed that trial models for the Toy Story and Monsters, Inc. characters were at one point originally set to appear in Kingdom Hearts II, with negotiations from Pixar to have them appear in the game before they were shelved.

Kingdom Hearts
Monstropolis encompasses all of the distinctive central locations of the Monsters Inc. factory. The world begins in the Robī & Ofisu (ロビー＆オフィス) area. The initial area is a large circular hub-like area which features dual maps of the world over-looking it at opposite ends. Behind this area is access to scare floors K-P. Proceeding though a few small hallway sections leads to Warai Furoa (笑いフロア), marked off as floor F and color in green. The Laugh Floor is spacious room containing office departments on the left and door stations on the right, which allow monsters to enter the human world. Accessing the door station between Mike and Sulley allows for Sora to access the Doa Sōko (ドア倉庫).

Sora accesses the door vault by sliding on the rails using flowmotion though defensive measures must be taken to avoid Heartless attacks as well as electrical malfunctions. Initially, the party arrives at the Sōko: Jōbu (倉庫:上部) though the next area and rail sliding section must be access by entering a door on the wall. At the next upper level section is a yellow door which leads to the door vault's Sōko: Kabu (倉庫:下部). Entering through the brown door on the left side will take the party back to the upper levels, in which the rails can be used to return to the laugh floor. Continuing to the end of lower level leads to a white door which will take the party to Kōjō (工場).

The initial area of the factory is Kōjō: Chikashitsu (工場：地下室). Continuing to the next room leads to a yellow conveyor belt where initially the lasers must be dodged to continue. The next room is a series of hallways known as Kōjō: 1-Kai (工場：1階). At the end of this room is a yellow elevator which will take the party to Kōjō: 2-Kai (工場：2階). Through the next series of hallways leads to a yellow lighted room which exits into another yellow conveyor belt section. Exiting the factory leads to Hatsudensho (発電所).

Exiting through the initial brick hallways leads to the outside area known as The Power Plant. Reaching the top of the power plant known as the Renraku Tsūro (連絡通路) leads to a dive that allows exploration of the Tankuyādo (タンクヤード). The crane allows the party to re-access the accessway. Utilizing the rail way on the right side leads to an indoor structure which contains a battlegate. The Doa Sōko e no Uramichi (ドア倉庫への裏道) is directly after this and leads back to the Sōko: Chika (倉庫:地下) which is at the very bottom door vault area. Taking the elevator at the end of room will take the party back to upper level of the door vault.

Kingdom Hearts III
Sora, Donald, and Goofy arrive outside the building, but are freaked out by each other's monster forms. Sora is confused by their new looks and is further puzzled when they read the sentence "We Scare Because We Care". The trio walks into the building where they see two creatures with a child and Sora thinks that they're scaring her, but changes his mind after seeing her happy. Boo, Mike, and Sulley introduce themselves with Boo mistaking Donald for Mike Wazowski. Introductions are cut short when the Unversed appear, creatures that Sora has never seen before. Sulley and Mike get Boo to safety and decide to help the trio fend off the new legion.

Deciding to send Boo back to her world, Mike and Sulley lead the trio to the Laugh Floor. However, they find Unversed using the closet door portals to scare children for scream energy. Boo's door is sent back to the Door Vault in the process, forcing the heroes to find doors into the vault. Using stray doors to move around the vault, Sora, Donald, and Goofy find the Heartless have also appeared along with the Unversed. Eventually, they track down what they think is Boo's door, but end up inside the Factory's basement, where they meet Randall, Mike and Sulley's former banished coworker. Randall gloats about his impending success and return, then leaves as he shrugs off Sora's defiance.

Sora and company fight their way through the factory, fending off Randall's traps and minions along the way. Towards the exit, Randall attempts to sic a machine on to them, but fails. When they escape, they find the Unversed have set the surrounding buildings on fire and assist CDA Agents in extinguishing them. Eventually, they arrive at the Service Area, where Randall unleashes the machine, now animated into the Lump of Horror Unversed. The heroes defeat the Unversed before Sulley sends Randall back into the human world through the door whence he came.

Before Sora and company can return Boo to her door, the door is sent all the way back to the Laugh Floor. The group encounters Vanitas, whom Sora recognizes from his vision at La Cité des Cloches. Vanitas explains that he had come to Monstropolis to reconstruct his heart and had manipulated Randall. He also reveals to Sora the existence of Ventus, who is still inside Sora's heart, and prepares to extract Ventus's heart to complete his own. Sulley grabs Vanitas and throws him through several doors before shredding the last one. Sora, Donald, and Goofy then take their leave, bidding farewell to Mike and Sulley.

Later in San Fransokyo, when Fred removes his monster mask to reveal his human face, Sora is relieved and nearly expresses his belief that it was the same world as Monstropolis, but Donald and Goofy stop him from carelessly breaking the world order.

Sometime after Xehanort's defeat, Mike and Sulley return Boo to her door.

Characters

 * Maintenance are mentioned by Sulley.
 * Trailer Son and Mom do not appear in the game physically, but are heard.

Character Design
Sora, Donald and Goofy turn into monsters when they visit the world.