Sirius is a luminary, one of the cosmic entities (embodied in stars) that rule the universe. He begins the book on trial for his celestial life, accused of murdering a lesser star with a Zoi (an Infinity Gem sort of thinger). Sirius has a rubbish defense and, being known for his terrible temper, the court finds against him. The Dog Star is shunted to Earth and reborn as a normal dog. If he can find the missing Zoi, Sirius will be allowed to re-ascend. If not, well... this dog will go to heaven the old fashioned way.
Sirius' dog life is far from a Disney movie. His first memories are starvation and then, horribly, being chucked in the river with his litter mates. He's saved by a little girl, Kathleen, only to join in the misery of her life. Kathleen is part Irish and virtually an orphan. Her mother has fled overseas (not wholly explained) and her father is in prison for his involvement with the IRA. Kathleen is bullied by virtually everyone in her small town, especially her domineering, selfish aunt, Duffie.