THURSDAY DECEMBER 19

It was an old dog with stiff joints, damaged by calcification. Illness had caused the animal to resemble a hyena, for its powerful chest and strong neck shrank abruptly into a skinny backside, with the tail curling around its testicles.

The mangy animal came and went. No one could remember when it had first appeared. In a way, it belonged to the district: an unpleasantness impossible to avoid, like the noise of the trams, double-parked vehicles, and untreated sidewalks. People had to take precautions, keep basement doors closed and locked, haul cats in for the night, and secure lids tightly on garbage bins in back yards. Now and again someone complained to the public-health authorities when food scraps and other items of rubbish were left scattered beside bicycle racks three mornings in a row. They rarely received any response, and nothing was ever done to catch the beast.

If anyone had stopped to consider how the dog actually lived, it would have been obvious that it moved around the neighborhood according to a pattern, out of step with the calendar and therefore difficult to spot. If anyone had taken the trouble, they would have realized that the dog was never very far off, and that it seldom roamed beyond an area measuring just fifteen or sixteen blocks.

He had lived like that for almost eight years.

He knew his territory and avoided other animals as much as possible, giving a wide berth to lapdogs on gaudy nylon leads, and had understood long ago that pedigree cats with bells around their necks were a temptation best resisted. He was a stray mongrel in Oslo’s upper-class west end and knew how to lie low.

The period of mild weather in early December was over, and now a biting pre-Christmas frost had glazed the asphalt. There was a hint of snow in the air. As the dog’s claws scratched the black ice, he dragged his back leg behind him. The glare from the street lamp highlighted a gash on his left haunch, liver-colored on his close-cropped fur and speckled with yellow pus. It had snagged on a spike the previous evening while he was searching for somewhere to sleep.

The apartment building was secluded, set back from the street. A paved walkway divided the front garden in two, and knee-high chain-link fencing, painted black, enclosed the wet, dead grass and a flowerbed covered with tarpaulin. A twinkling, decorated Christmas tree flanked either side of the entrance.

This was the dog’s second attempt to gain entry in the course of the evening. There was usually a way. Unlocked doors were easiest, of course. A quick leap, a swipe of the paw at the door handle. It was usually immaterial whether the door opened in or out: unlocked doors were a piece of cake. But rare. Normally he had to search for basement windows opened a chink, loose boards around walls due to be repaired, or gaps under rotting cellar stairs. Access points that everyone else, apart from him, had forgotten. They were not to be found everywhere and sometimes these gaps were me