Jillian Zinman despises the seemingly lifeless doll, Mary-Ellen, due to her sisters ignoring her in favor of the toy and for claiming their pranks were ideas that the doll told them to do (e.g. giving Jillian a haircut while she is sleeping). Jillian tries to think up revenge plans against the twins, but she's also perfectionist who dismisses ideas and is shown to care for her sisters regardless of the mistreatment.
Jillian wants to earn extra money performing at birthday parties. Her initial plan is to put on a clown act for her first job. She forgets her promise to take Katie and Amanda Zinman to see a local ventriloquist act at the Little Theater, making plans with her best friend Harrison Cohen. However, her mother forces Jillian to honor her promise and suggests that watching the show might give her ideas for her clown act. The twins insist on taking Mary-Ellen despite Jillian's protests, and Harrison comes along.
The four kids see the ventriloquist act starring a young, possibly teenage, ventriloquist Jimmy O'James, and his dummy Slappy. Jillian is unimpressed with Slappy's humor, claiming in Chapter 2 that his act left her bored to tears, but she is one of the few to notice that Jimmy is upset with Slappy's performance. Katie and Amanda are called up to meet Slappy, and they bring Mary-Ellen on stage. Slappy insults all three, making Jillian wonder why Jimmy is having his puppet insult her sisters. Jillian tries to comfort her sisters when they return to their seats, but the girls decide to run back stage and force Jimmy to apologize for Slappy's insults.
Jillian and Harrison run after them and are forced to split up when the hallway divides in two. Jillian takes the darker route and finds herself outside Jimmy's dressing room while he argues with Slappy. She witnesses Slappy hitting Jimmy, but she is discovered by Jimmy. Jimmy pretends that he accidentally hits himself with Slappy, but this marks the beginning of Jillian's antagonistic feelings toward the dummy, which Mary-Ellen later exploits. Jillian apologizes to Jimmy for distubring him, and both Jimmy and Slappy tell her, "No problem."
Jillian takes her sisters and Mary-Ellen to Dairy Queen afterward. She buys the twins and the doll their own ice-cream cones, but the girls ignore her and talk to Mary-Ellen the entire time. Jillian cleans all three's messy faces with napkins. At her house, Jillian is surprised to see Slappy sitting on the couch in her living room. However, Harrison pops out from behind the couch, teasing her for believing the dummy was alive. Harrison had discovered Slappy (now damaged) in an open trash can on the side of the Little Theater. He leaves Slappy with Jillian because her father is a hobby carpenter with his own workshop in the basement. Since Mr. Zinman likes repair jobs, Harrison hopes he'll repair Slappy. (Mr. Zinman agrees to look Slappy over, but only after he finishes a coffee table he had been working on for six months. This keeps Slappy in Jillian's house for the rest of the book despite him belonging to Harrison.) Jillian and Harrison discover the piece of paper with the magic words in Slappy's pocket, but Jillian doesn't finish reading them when she's called to dinner.
In the TV version, Slappy shows up at the Zinmen house, and Jillian takes care of the dummy, she often gets annoyed when her sister talks about Mary-Ellen. because that's all they talk about. For the past few days, there have been some cruel pranks around the house, like finding Jillian's pet lizard in Slappy's mouth, she asumes the twins are responsible.
One night when the siblings invite their friend's Harrison and Eddie over, and they discover that Slappy is alive as he drags Eddie across the floor, "This is My Party Now! Slappy screamed, "I Want My Bride!" Jillian gives him Mary-Ellen, but Slappy knocks it away, and says that he wants Jillian to be his Bride, Mary-Ellen comes to life and reveals that she was the one who brought Slappy to life, and she tries to convince Slappy to be her husband, he refuses, then the doll and dummy fight, Slappy pushes her in a saw, and she pulls him in and they both get sliced up in pieces. In the end, Jillian hopes that Slappy is gone forever, but then, Slappy's spirit possesses Jillian, her eyes turn green and she has Slappy's voice.