Jessi knows a secret language! She learned it from Matt Braddock, the Baby-sitters' newest charge. Matt's been deaf since birth, and he uses sign language to speak. Since Jessi's Matt's baby-sitter, she has to use sign language, too.
Soon all the kids in Stoneybrook want to learn to sign... which keeps the Baby-sitters busy. Jessi's the busiest of all: she's working on another super secret, just for Matt.
Will Jessi be able to keep the secret and pull off her special event? Of course she will -- she's a member of the Baby-sitters Club!