Sirius is a dog with humanlike intelligence. He is raised by scientist Thomas Trelone in North Wales, near Trawsfynydd. Thomas embarks on a program of using steroids and other chemicals to rapidly develop cognitive power of dogs, resulting in super sheep dogs. Sirius, however, proves to possess a dog intelligence comparable to a normal human being, as he is able to communicate with English words, although it takes some time for the humans to understand his canine pronunciation. He is born at the same time as his creator's human daughter, Plaxy, and the two of them are raised together as brother and sister. When Plaxy leaves for school they drift apart and troubles for Sirius begin. With human intelligence and emotion he declines to lead a life of a regular dog and he tends to develop as a person, but keeps running into obstacles.