The heatless sunlight was flooding the lonely white road when he began the homeward journey. He was very tired, for he had slept little and his sleep had been broken by the recurring fantasy of rescue.

The silence was suddenly broken by the distant thunder that came rolling over the white roofs. It came—there could be no doubt now—from beyond the northern hills that had once been London’s playground. From the buildings on either side little avalanches of snow went swishing out into the wide street; then the silence returned.

Professor Millward stood motionless, weighing, considering, analysing. The sound had been too long-drawn to be an ordinary explosion—he was dreaming again—it was nothing less than the distant thunder of an atomic bomb, burning and blasting away the snow a million tons at a time. His hopes revived, and the disappointments of the night began to fade.

That momentary pause almost cost him his life. Out of a side-street something huge and white moved suddenly into his field of vision. For a moment his mind refused to accept the reality of what he saw; then the paralysis left him and he fumbled desperately for his futile revolver. Padding towards him across the snow, swinging its head from side to side with a hypnotic, serpentine motion, was a huge polar bear.

He dropped his belongings and ran, floundering over the snow towards the nearest buildings. Providentially the Underground entrance was only fifty feet away. The steel grille was closed, but he remembered breaking the lock many years ago. The temptation to look back was almost intolerable, for he could hear nothing to tell how near his pursuer was. For one frightful moment the iron lattice resisted his numbed fingers. Then it yielded reluctantly and he forced his way through the narrow opening.

Out of his childhood there came a sudden, incongruous memory of an albino ferret he had once seen weaving its body ceaselessly across the wire netting of its cage.