At the start of the half term holiday, the most important thing Thomas has to worry about is when, or if, he'll stop hiding his hearing aid. But when he and his twin sister, Bianca, find a dog in their local park, their mother gives them just one week to find its real owner before she takes it to a dogs' home. When the twins begin to suspect that the lady who replied to their advert isn't who she claims to be, and determined to reunite Rocky with his real owner, they begin an investigation that will lead them to a gang of pet thieves. Can they convince the police of their findings and rescue Rocky before it's too late?