In the year 311 A.C. the Night's Watch failed in its duty. The Horn of Joramun was blown and the Wall, which has stood since the Long Night, fell. Hordes of invading White Walkers and Others swept the land. They sought out and exterminated anything that lived. They had no fear, no mercy, no pity, and no remorse.
For a generation, pockets of humanity resisted. From Winterfell, to Sunspear, to Braavos, and Asshai, people fought back. Then in 343 A.C. the humans had victory in their sights. They had shattered the wave of the undead against their own stronghold. Humanity had pushed the Others back into the land of Always Winter. They had won.
But the Others would not accept defeat. Using dark magic not seen since the dawn of existence, they had sent a creature into the past. The resistance learned of this and sent one of their own after the creature. For the final battle for the fate of the world would not take place in the future. It would take place here, in the present day.