Upon arriving at Frankenollie's residence, Mickey is dropped down a trapdoor into the scientist's laboratory, where he reveals his plan to switch his donor's brain with that of his enormous Pete-resembling monster, Julius.

As Julius continues to pursue Minnie and a battle between him and Mickey occurs, they land on a telephone line and become electrocuted, sending their brains back into their proper bodies.

[3] Bailey at first saw approval from studio president Jeffrey Katzenberg and Disney Animation executives Thomas Schumacher and Peter Schneider regarding the rework of an idea he had for a Roger Rabbit short, Tourist Trap, with Mickey and Donald Duck heading on a vacation and Donald attempting to kill Mickey, but after a failed storyboard screening, Bailey received permission from Roy E. Disney to develop another short idea, Runaway Brain.

[3] Jim Beihold was assigned to layout, Ian Gooding served as art director, and Andreas Deja, who had animated Scar in The Lion King (1994), developed the monstrous version of Mickey possessed by Julius based on Bailey's sketches.

[4] The general plot of the cartoon refers back to Mary Shelley's Frankenstein, with Doctor Frankenollie and Julius being heavily inspired by the titular character and his monster.

In addition, Mickey whistles the short's primary theme, Steamboat Bill, before he is dropped down into Frankenollie's laboratory and a Polynesian-styled rendition of it is also played in the credits.

[5] In terms of general reception, the macabre nature of the animation's plot brought criticism from some Disney fans due to the contrast with the previously light tone of Mickey Mouse cartoons.

Andy Mooney, then chairman of Disney's consumer products unit, remarked to the Los Angeles Times in 2003 that "the very fact that Mickey was possessed was very disturbing" to some audiences, though the character "overcomes that".

[6] It was first released in North America on August 11, 1995, alongside theatrical showings of A Kid in King Arthur's Court, then on September 12, 1996, with The Hunchback of Notre Dame in Australia and on October 18, 1996, attached to A Goofy Movie in the UK.