Through a misunderstanding, Junie B. thinks that her new baby brother is really a baby monkey, and her report of this news creates excitement and trouble in her kindergarten class.
Junie, a spunky, sometimes exasperating, kindergartener, looks forward to winning lots of prizes at the school carnival, but a fruit cake was not exactly what she had in mind.
Junie B is very upset when a boy in her class plans to invite everyone except her to his birthday party, but her grandfather helps her deal with the situation.
Frustrated because the rules for her class's Pet Day will not let her take her dog to school, Junie B. Jones considers taking a raccoon, a worm, a dead fish, and other unusual replacements.