Polly and her friend Rex are not just a girl and a dog. More precisely, Polly is a completely ordinary girl, and Rex is the guardian spirit of the ancient Penhallow mansion. And this whole huge mansion is full of dogs: they poke their faces out of bas-reliefs and balusters, look from paintings, sit like statues and figurines. They all once lived here in Penhallow and forever remained its guardians. And one such magical dog is so sad that even Polly felt it. A long time ago, a little girl forgot a toy in her room, but the dog did not forget his owner. Polly and Rex don't know what to do, how to get the dog out of his sadness. Is it possible to find a girl who is just as sad and who could be helped by a dog?