Eleven-year-old Sally Jane Hesslop loves her Dad, her hamster Melvin, and her best friend Katie. Her four-year-old brother Robbie she's not so sure about. She tries to love him, but Robbie eats dirt and wakes Sally up every morning by hitting her on the head with a rubber spatula. Robbie's a hard person to love.

But that's okay. Who needs little brothers when you have a fluffy orange hamster like Melvin? Melvin's got Super-Secret-Agent training, a T-Shirt of Invisibility, and his best friend is Arnold the Iguana, the mascot of Sally's fifth-grade class.

With Melvin to save the day, Sally and Katie know everything will turn out okay.