The series is set in the mythical land of Westeros that is divided into seven kingdoms; namely, The Kingdom of the North, The Kingdom of the Isles and Rivers, The Kingdom of the Mountain and Vale, The Kingdom of Rock, The Kingdom of the Reach, The Kingdom of the StormLands, and The Kingdom of Dorne, each kingdom is ruled by various houses.
The ruler of the seven kingdoms is King Robert Baratheon who has won the throne after conquering Rhaegar Targaryen (popularly known as Robert’s Rebellion). He marries Cersei Lannister in order to acquire the loyalty of the Lannister House. He names his loyal friend Eddard Stark as the ‘Hand of the King’.
Pandemonium breaks lose when King Robert dies and Joffrey, his first son is enthroned. Joffrey gets Eddard executed (who confesses to treason to protect his family), inviting the wrath of the House Starks. However, it is later known that, the children of Cersei are actually born out of an illicit relationship with her twin brother Jaime Lannister. This provokes the deceased King Robert’s brothers―Stannis and Renly―to claim the throne.
Thus, the war of five kings―Joffrey Baratheon, Stannis and Renly Baratheon, Robb Stark (son of Eddard Stark) and Balon Greyjoy―is the underlying theme of the series. Meanwhile, the Targaryen siblings―Prince Viserys and Princess Daenerys―are preparing to reclaim their fallen throne.