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 the former using the pharmacy's stock of costumes. While trembling in fear, he is suddenly attacked by an ogre mask, which transforms him into an Igor-esque ogre that immediately becomes Slappy's right-hand monster.
Walter accompanies Slappy to the Tesla Tower, where he uses his magic to reactivate the machine and bring every Halloween decoration in Wardenclyffe to life. He also impresses 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 arrive at the Tesla Tower disguised as a Pumpkin Head, a witch, and a skeleton, Walter spies on them before revealing himself as they confronted Slappy. He brings his master Haunted Halloween and prepares the tower so that Slappy could destroy the book. However, Sonny and Sam cause a chain reaction to overload the tower and blow up the fuse. As Sarah sucks in all the monsters inside Haunted Halloween, the Ogre Mask is removed from Walter, transforming him back to normal.
Apparently, his transformation also erased any memory of him being a monster, as pointed out by Stine. Over the next few months, Walter begins going out with Kathy.
- Walter and his ogre mask form both appear in Goosebumps HorrorTown, originally the Haunted Halloween event and another thing is that he appeared to have a romantic attraction for Mrs. Dark.
- Walter the Ogre appears in Goosebumps Dead of Night as Slappy's henchman, just like in Haunted Halloween, except he has clones this time.