Brian Daly, one of the stars of the play "Death of a Salesman," exits a theater with two co-stars after a performance. But Daly is distracted by the sound of a girl’s soft sobbing. Curious, Daly follows the sound and finds a blonde little girl leaning against the wall and crying. Daly kneels and asks if she is lost. The girl, shielding her face with her hands, claims that her brother has left her by herself. Daly offers her a handkerchief. The girl takes it and thanks Daly, finally revealing her face to him. A shocked expression crosses his face, but before he can react, he is knocked out-cold by a silhouette from behind. The girl tip-toes around him, apologizing for "playing rough."
The next day, Batman and Robin are browsing through photos of missing actors from a 10-year-old sitcom "Love That Baby." Daly is the latest to disappear. The blond girl, it turns out, is Mary Louise Dahl, the show's child star "Baby-Doll." According to Batman, Dahl suffered from a severe systemic hypolasia, a rare disease that prevented her body from aging. Though now in her mid or late 20's she still has the body of a small child. They are interrupted by a radio call from Detective Bullock, requesting immediate backup in stopping an attack at the same theatre where Brian Daily disappeared from. Batman and Robin arrive, but are unable to stop the attackers from kidnapping another ex-actor from the show, Tammy Vance who is also appearing in "Death of a Salesman". They try to give chase in the Batmobile but Baby throws herself in front of the car, forcing Batman to avoid her. The Batmobile crashes, and the attackers manage to escape.
Robin holds a crying Baby-Doll in his arms while the girl keeps asking for "her mommy." A red-haired woman emerges from the crowd and takes the girl into her arms, seemingly scolding her. Baby replies with her trademark line from the TV show, "I didn’t mean to," and Batman and Robin realize who she is. Baby-Doll throws a smoke (basketball) bomb and disappears in the smoke along with her "mommy."
Robin has reviewed the history of the show, including its end. In an attempt to boost falling ratings, the producers introduced a new character, "Cousin Spunky." Feeling that Spunky was stealing her spotlight, Dahl quit the show, forcing its cancellation, and attempted to launch a career as a serious actress. The attempt failed, since her condition made it impossible for people to take her seriously. She has not been heard from since.
Batman and Robin realize that the actor who played Spunky is the only one still at large, and form a plan to trap Baby.
Spunky, now in his early to late twenties, is forming a band in his garage, when he is kidnapped by Dahl's thugs. He is taken to her hideout, which has been decorated into a mock-up of the show's set. As the actors protest – pointing out that, after all, it was her fault that the show was canceled – she angrily retorts that her life after the show was a failure, so she's "going back" to the way things were before.
She plans to re-enact a birthday party episode, only with dynamite planted in the cake. But at the last second, "Spunky" grabs the dynamite in his mouth and flings it away, saving the party and revealing himself to be Robin in disguise.
Enraged, Baby-Doll points her doll, Mr. Happy-Head (a disguised machinegun), at "Spunky," when Batman makes his entrance, disarming both Baby and her guards. But then Baby's "mommy" who is revealed to be Baby's right hand woman named Miriam attacks Batman and Robin with her impressive martial arts skills, giving Baby the chance to run away. The duo is quick to dispose of the woman and Batman continues to pursue Dahl while Robin frees the actors and guides them to safety.
The pursuit takes the pair into the "Funland" amusement park, where Dahl ends up in the Mirror House. As she lies in wait for Batman, Dahl sees herself in the various trick mirrors. One of them elongates her reflection so as to make her appear a grown-up adult – but this, she realizes, is just as much an illusion as the recreation she was trying to make. Breaking down, she begins shooting mirrors at random, finally ending with her own reflection. Batman appears and takes the doll from her unresisting hand. She cries as she clutches tightly at Batman's leg, he placing an understanding hand on her head at her words: "I didn’t mean to."