The musical is based on Sendak's books Chicken Soup with Rice, Pierre, One was Johnny, Alligators All Around (which comprise 1962's The Nutshell Library), and The Sign on Rosie's Door (1960).
Sendak based the story on a demonstrative little girl who used to sing and dance on the stoop of her building, whom he observed while he was a little boy growing up in Brooklyn.
Rosie, the self-proclaimed sassiest kid on her block of Brooklyn's Avenue P, entertains everyone by directing and starring in a movie based on the exciting, dramatic, funny (and slightly exaggerated) story of her life.
[1] It was directed by Maurice Sendak, animated by Ronald Fritz and Dan Hunn of D&R Productions Inc., with Carole King voicing the title character.
Rosie and the Nutshell Kids (Johnny, Alligator, Pierre, and Kathy) live on the same block on Avenue P in Brooklyn, New York.
The stage version of Really Rosie follows a similar plot to the animated special, with a few key changes: Jennings, Coleman A. and Maurice Sendak.