X-rays cause a litter of puppies to be born mutants, with hugely oversized heads. All but one are killed as 'monsters', but the remaining one, named Socrates, grows to be very intelligent–even learning to speak. However, it is still a dog, and will not abandon its abusive master, which in the end leads to its demise. The story ends on the question of whether the dog's intelligence will be passed on to its pups.
Totally predictable. The only interesting bit is how the dog, despite its intelligence, is trapped by the loyalty which we associate with the species.