A long time ago, the sorcerer Yen Sid used a magic paintbrush to create a world for Disney's forgotten characters (from creations to rusted Disney attraction parts). This was held in an intricate model on a table and was accessible through a mirror. One day Mickey taking a nap, when Mickey's mirror makes an odd noise, awakening him. He climbs onto Mickey's dresser and pokes it, to see that he can go through it. Mickey, who finds himself inside Yen Sid's study, sees the model and waits until Yen Sid is gone before going back to take another look at it. Fiddling with it, Mickey attempts to create a statue of himself in the world but instead creates the Shadow Blot. Mickey attempts to erase it, and ends up spilling paint on the model. He attempts to clean it up in a haste, but just spills even more paint and thinner, twisting the world. Mickey managed to escape without Yen Sid knowing, returning to his world where he went on to live years of success and fame, but his meddling had consequences.
The paint and paint thinner have severely damaged the world and created an evil being called the "Shadow Blot", who has usurped power from Oswald the Lucky Rabbit, Disney's first creation before Mickey, and sent the world into ruin. A long time later, Mickey is kidnapped by the fake Shadow Blot and sent into the world. Oswald is shocked to see them and flees the scene.
The Mad Doctor worked closely with Oswald (at first) then the Blot, creating robotic "Beetleworx", which once helped out but now patrol the wasteland, capturing Gremlins, robot versions of Mickey's friends called "buddies" for Oswald, looking like Donald Duck, Goofy and Daisy Duck as Oswald misses popularity. When Mickey escaped, he found out what he had done. To help clean up his mess, he pulled in the paintbrush he got kidnapped. So now, Mickey gain the trust of Oswald and save the forgotten world.
- Ben Small as Thomas, Toby, Ferdinand and Rheneas
- Tara Strong as Twilight Sparkle and Ortensia
- Keith Wickham as James, Percy, Skarloey, Sir Handel, Den and Dash
- Matt Wilkinson as Rusty, Diesel 10 and Bash
- Steven Kynmann as Peter Sam
- Michael Legge as Luke
- Repurt Degas as Dart
- Kerry Shale as Diesel
- Ashleigh Ball as Applejack and Rainbow Dash
- Andrea Libman as Fluttershy and Pinkie Pie
- Tabitha St. Germain as Railty
- Cathy Weseluck as Spike
- Bret Iwan as Mickey Mouse
- Frank Welker as Oswald the Lucky Rabbit, the fake Shadow Blot and the Shadow Blot
- Bob Joles as Gremlin Gus
- Corey Burton as Yen Sid
- Jim Cummings as Pete, Small Pete, Pete Pan and Petetronic
- ???? as The Mad Doctor
- ???? as Discord
- Skarloey, Rheneas, Sir Handel, Peter Sam, Rusty, Devious Diesel, Diesel 10, Discord, Den and Dart guest starring in this film.
- Thomas and Twilight Sparkle's Adventures Team meets Oswald the Lucky Rabbit, Mickey's oldest brother, for the first time in this film.
- Diesel, Diesel 10, Den and Dart was helping the Mad Doctor.
- Discord was helping the real Shadow Blot to steal Mickey's Heart.
- This movie is for in the honor of Epic Mickey 2: The Power of Two, a sequel to Epic Mickey.
Happy Ending (only and no sad ending)
- Small Pete starts working good with all of the Gremlins. (for showing his ship log to Gremlin Bennett)
- The Gremlins dance to "It's a Small World" and play with the Clock Tower watching them nearby. (for redeeming the Clock Tower)
- Pete Pan fights Captain Hook and Captain Hook slashes him, then laughs heartily of having his adversary putting up in good spirits fighting him (for saving his Sprite and not killing Hook/smashing him to bits).
- Petetronic and Gremlin Nova, Gremlin Sparks, and the other three Gremlins in Tomorrow City ride on the Wasteland version of Astro Orbitor (called the Rocket Ride by Gremlins). (for redeeming Petetronic)
- Big Bad Pete gives away the fireworks to the citizens of Mean Street. (for completing his "Bunny Roundup" quests, as well as helping Small Pete, redeeming Petetronic, saving Pete's Pan Sprite and recovering the Colonel Pete cartoons)
- The Mean Street citizens give Horace praise for his success at being a detective. (for solving his cases)
- Animatronic Daisy, outside the Tiki Hut in Ventureland, grabs a tiki mask that looks like Donald and holds it dearly. (for finding all of her parts)
- Animatronic Goofy cleans OsTown with his vacuum leg while hopping on his other leg. (for finding all of his parts)
- Animatronic Donald is talking to a Gremlin in Bog Easy, until a Lonesome Ghost appears and spooks him. Donald angrily squawks at the ghost for scaring him while the Gremlin laughs. (for finding all of his parts)