After almost a decade-long summer, Westeros now braces itself for an equally-harsh winter, in more ways than one.
In King's Landing, Cersei Lannister has finally achieved her lifelong goal. In one swift move, she has practically wiped out all of her enemies, rivals and obstructions, including Queen Margaery Tyrell, Lord Mace Tyrell, Loras Tyrell, her uncle Kevan, Grand Maester Pycelle, the High Sparrow and the entirety of the Faith Militant, by destroying the Great Sept of Baelor using large amounts of wildfire. Her three children, Joffrey, Myrcella and Tommen are all dead, Tommen having taken his own life shortly after the Sept's destruction, and thus Cersei has crowned herself the undisputed Queen of the Seven Kingdoms, with Qyburn and the undead knight Ser Gregor Clegane at her side. After years of serving as the power behind the throne, House Lannister has now become the new royal house. However, Cersei is still haunted by the prophecy she was told about during her childhood, which claimed that the deaths of her children would only predate her own, making her all the more determined to crush her enemies once and for all.
Despite Cersei's rise to power, the War of the Five Kings continues to ravage the country.
In the Riverlands, the last bastion of King Robb Stark's independent Northern/Riverland Kingdom, Riverrun, has fallen to House Lannister and House Frey, led by Ser Jaime Lannister. Robb's great-uncle, Brynden "The Blackfish" Tully, is dead, having chosen to fight to the death rather than surrender Riverrun, a move that was made possible by his own nephew, Edmure Tully, who ordered the Tully garrison to stand down for the sake of his wife and unborn child. Despite the victory of House Frey, Lord Walder Frey is also dead, having been murdered by Arya Stark for his role in the Red Wedding, leaving the rule of the Riverlands in unknown hands. Also, Sandor "The Hound" Clegane has survived his brutal fight with Brienne of Tarth and attempted to build a new, humble life, but has been dragged back into conflict by the Brotherhood Without Banners, who intend to head North to fight in the war that is coming. Brienne and her squire, Podrick Payne, were only barely able to escape the fall of Riverrun, following their failed attempt to recruit the Blackfish to their cause.
On the Iron Islands, King Balon Greyjoy is dead, having been murdered by his psychopathic brother, Euron Greyjoy. Balon's son, Theon, who returned to the Iron Islands after helping Sansa Stark escape from Ramsay Bolton, endorsed his elder sister, Yara, as their father's successor at the subsequent Kingsmoot, but Euron has usurped the Salt Throne with the promise of an alliance with Daenerys Targaryen across the Narrow Sea to help him conquer the Seven Kingdoms. In desperation, Theon and Yara have stolen most of Euron's fleet and sought out Daenerys's help first, sealing an alliance between House Targaryen and House Greyjoy to help Daenerys retake the Iron Throne and execute Euron for his crimes. Euron, however, has ordered the construction of an even bigger fleet to destroy all who may stand in his way.
Also aiding the Targaryens are the Martells. In Dorne, Ellaria Sand and the Sand Snakes have staged a coup and murdered Prince Doran Martell and his heir, Trystane, and taken control of Dorne in retaliation for Doran refusing to avenge his brother, Oberyn. With Cersei declaring war against the Martells for the murder of Myrcella, Ellaria has turned to Daenerys and her loyalists to bring the Lannisters down. Lady Olenna Tyrell, in response to the deaths of her son, grandson and granddaughter, has also turned to the Targaryens and Martells through Daenerys' loyalist, Varys.
Most of the strife in the Seven Kingdoms, however, lies at the North. At Winterfell, Roose Bolton is dead, having been murdered by his own son, Ramsay, along with his wife and newborn child, leaving Ramsay as the new Warden of the North and enabling him to solidify alliances with House Karstark and House Umber, with Rickon Stark as a hostage. At the Wall, following a horrific mutiny, the deceased Lord Commander Jon Snow has been revived by the Red Priestess Melisandre, while Sansa Stark has escaped from Winterfell and Ramsay, to seek shelter with Jon. Having resigned his post in the Night's Watch, Jon has built an army to retake Winterfell from Ramsay, aided by Ser Davos Seaworth, Tormund and Lady Lyanna Mormont, and though they have achieved victory and destroyed House Bolton thanks to the Knights of the Vale, courtesy of Lord Petyr "Littlefinger" Baelish, House Stark's future is left in a questionable position. With Robb and Rickon dead and Bran believed dead as well, the Northern Lords have turned to Jon and named him the new King in the North, while Sansa has rejected Littlefinger's proposal to help him take the Iron Throne and serve as his queen, setting the stage for conflict between Jon and Littlefinger. Melisandre, meanwhile, has been dismissed from Jon's service following the discovery of her role in Shireen Baratheon's death, stripping Jon of easily one of his most valuable advisors, and is now heading to locations unknown.
In the Reach, despite having to brave various obstacles, Samwell Tarly and his surrogate family, Gilly and her son, have finally reached the Citadel in Oldtown, where Sam intends to train as a maester to gain some insight into the White Walkers and how to defeat them.
Across the Narrow Sea, after having finally defeated the Slave Masters and the Sons of the Harpy and acquired an army made from the Dothraki, the Unsullied, House Greyjoy, House Martell and House Tyrell, Queen Daenerys Targaryen has finally set sail for Westeros, with Tyrion Lannister at her side as her Hand of the Queen, though at the cost of two of her advisors: Jorah Mormont whom Daenerys has sent to find a cure for his spreading greyscale infection, and Daario Naharis, her lover whom Daenerys has ordered to stay behind so that she may pursue a marriage alliance to aid her in her campaign. Little does she know, however, is that she is not be the last of her family after all, and there is somebody across the Narrow Sea who could legitimately challenge her claim to the Targaryen monarchy.
Also, in Braavos, though Arya Stark has regained her sight and favor with the Faceless Men, her code of honor has put her at odds with the guild, causing her to botch an assassination attempt on an innocent target. Despite almost being killed, Arya has escaped Braavos with her life and returned to Westeros, beginning her vengeance for her family by murdering Walder Frey and his sons, Lame Lothar and Black Walder, for orchestrating the Red Wedding that claimed the lives of her brother and mother.
Beyond the Wall, Bran Stark has been training in his powers in greensight under the tutelage of the Three-eyed raven and learned some devastating secrets, such as the true parentage of his alleged half-brother, Jon Snow: he is the son of Lyanna Stark, Ned Stark's sister, and possibly fathered by Rhaegar Targaryen, which would make him the nephew and a rival claimant to Daenerys Targaryen.
Following an attack which claims the lives of the Three-eyed raven, Hodor and the last of the Children of the Forest, Bran and Meera Reed have been rescued by Bran's undead uncle, Benjen Stark, and taken back to the Wall to prepare for the arrival of the Night King, who at all costs must be defeated before all Westeros can do is dream of spring.