A family in Kristy's neighborhood gets a foster child, and Kristy is really excited. Lou, short for Louisa, turns out to be a real terror, and Kristy is having trouble handling her.
The baby-sitters are eager to start their animal observation projects for science class, at the Bedford Zoo. But on their very first visit, Mary Anne and her friends get involved in a mystery--someone has been letting the animals out of their cages.
Claudia's participation in the Baby-sitters Club is curtailed when Grandmother Mimi suffers a stroke and Claudia finds herself "Mimi-sitting" and fighting more frequently with her sister.
Now the Stacey and Jeremy have broken up, Claudia hopes that Jeremy will want to go with her to a school dance, but when a mix-up causes her to ask the wrong guy, she discovers that Jeremy may not be the boy she wants.