Lulu's parents refuse to give in when she demands a brontosaurus for her birthday and so she sets out to find her own, but while the brontosaurus she finally meets approves of pets, he does not intend to be Lulu's.
Lulu needs help from a boy named Fleischman if she is to earn money walking her neighbors' dogs, and she finds out that if she wants her business venture to succeed, she has to be nice.
When Lulu's parents go on vacation, the formidable Ms. Sonia Sofia Solinsky comes to babysit and Lulu behaves as badly as possible to get her to leave until Ms. Solinsky reveals her secret.