Rex the dog is all alone; nobody to feed him, nobody to walk him, locked in an old house he is miserable and waiting to die. Nobody knows he's there, nobody knows he's ill, and nobody cares...well...almost nobody. Healing Rex is the story of some extremely unlikely heroes; two fleas, Hooper and Ping, who have made their homes on Rex's back. Risking their lives on a journey into the unknown they venture inside the huge dog's body in order to try and find out what's wrong with him. Once inside they encounter all sorts of bizarre characters and creatures, both hostile and friendly, in their quest to mend the old dog's broken heart.