On Halloween night, he was working on sudoku puzzles when the pharmacy started acting up. He then found Slappy the Dummy, the Abominable Snowman, the Werewolf of Fever Swamp, and other monsters that had been brought to life by Slappy using the pharmacy's stock of costumes. While trembling in fear, he was suddenly attacked by an ogre mask and transformed into an Igor-esque ogre that immediately becomes Slappy's right-hand monster.
Walter accompanied Slappy to the Tesla Tower, where he used his magic to reactivate the machine and bring every Halloween decoration in Wardenclyffe to life. He also impressed his master by constructing a wooden chair with metal cuffs before relaying Slappy's message to have the other monsters retrieve both Kathy and Haunted Halloween.
When the kids arrived at the Tesla Tower disguised as a Pumpkin Head, a witch, and a skeleton, Walter spied on them before revealing himself as they confronted Slappy. Bring his master Haunted Halloween, he prepared the tower so that Slappy could destroy the book. However, Sonny and Sam caused a chain reaction the overloaded the tower to Walter's panic. As Sarah sucked in all the monsters inside Haunted Halloween, the Ogre Mask was removed from Walter, transforming him back to normal.
Apparently, his transformation also erased any memory of Walter being a monster, as pointed out by Stine. Over the next few months, Walter began going out with Kathy.
- Walter and his ogre mask form both appear in Goosebumps HorrorTown, originally the Haunted Halloween event.
- Walter the Ogre appears in Goosebumps Dead of Night, playing a role similar to his appearance in Haunted Halloween.