Jim Henson's 1986 film Labyrinth was at least partially inspired by the book. National Book Award for Children's Books, category Picture Books (hardcover).The theme of a protective sister is drawn from Sendak's own childhood, in which his older sister was his primary caregiver and devoted playmate. A child is stolen from its crib through a window, accessed by a ladder, and one of the illustrations of the lost baby is a deliberate portrait of the infant Charles Lindbergh Jr. Outside Over There draws more specifically from the Lindbergh case. That experience showed him the mortality and peril of children, which the adult Sendak has expressed in many books. In the documentary Tell Them Anything You Want: A Portrait of Maurice Sendak (2009), Sendak describes his awareness in 1932 (around age 4) of the sensational Lindbergh baby kidnapping case, including a newspaper photograph of the child's remains.

Ida then picks up her sister and heads home to her mother who has received a letter from her father where he promises to come home one day and asks Ida to watch over her sister. To find her sister among the crying babies, Ida plays a captivating tune on her horn until the goblins dance in a frenzy and fall into a stream. She does so and interrupts the goblins, now in the form of babies, in the midst of a wedding. She then hears her father's voice telling her to turn around into the rain. However, because she exits the window backwards she enters Outside Over There where she cannot find the goblins or her sister.

The changeling melts as Ida cradles it and Ida, realizing what has happened, blows her wonder horn, dons her mother's yellow rain cloak, and sets off after her baby sister.

One night while she is playing her horn and not paying attention to the baby, goblins sneak in through the window and steal her baby sister away, replacing her with a changeling made of ice. She plays her horn each night to make her baby sister sleep. The father of Ida, the main character, is away at sea. Outside Over There has been described by Sendak as part of a type of trilogy based on psychological development from In the Night Kitchen (toddler) to Where the Wild Things Are (pre-school) to Outside Over There (pre-adolescent). It concerns a young girl named Ida, who must rescue her baby sister after the child has been stolen by goblins. Outside Over There is a picture book for children written and illustrated by Maurice Sendak.

Sendak in the night kitchen