House of the Dragon is based on Fire and Blood, a novel by George RR Martin covering the rise and fall of the Targaryen dynasty as recorded by a maester in pre-Game of Thrones time. Since that book has been out for about two years, we have a pretty good idea of what we can expect to see from House of the Dragon’s plot.
The book picks up with Aegon’s Conquest, which marked the very beginning of a new world order, the establishment of King’s Landing as the world capital, and the physical formation of the Iron Throne.
Together with the help of his twin sisters, Rhaenys and Visenya (not to mention their three dragons), Aegon Targaryen I successfully subdued six of the seven kingdoms of Westeros, whether by conquest or their voluntary submission. Dorne was the only kingdom to successfully resist Targaryen rule. The only kingdom to prevail unconquered was Dorne, which Rhaenys left unharmed after finding its castles abandoned and only women and children occupying its seat.
From there follows a turbulent run of Targaryen kings and queens; good, evil, and everywhere in between; leading up to the great civil war between Aegon II and his half-sister Rhaenyra over their father Viserys I’s throne. That long struggle endured for two years, through which several large-scale battles were fought until both Aegon II and Rhaenya died mysterious and grizzly respective deaths.
The resulting power void fell to the young Aegon III to occupy, which resulted in the death of “the last dragon.” The end of Fire and Blood doesn’t lead directly into the events of Game of Thrones, closing out roughly 150 years before the fateful day Robert Baratheon shows up at King’s Landing to enlist Ned Stark as Hand of the King. The gap between the end of Fire and Blood and the beginning of Game of Thrones hasn’t been written about in depth, but Martin has said he plans on writing Fire and Blood part 2 when he’s finished the A Song of Ice and Fire series.
Game of Thrones followed the books closely and with little deviation for the first four seasons, after which substantial changes to the characters and plot were made at least partly due to the source work not being finished at the time. For those reasons, it’s hard to tell how faithfully House of the Dragon will tell the stories from Fire and Blood. With Martin at the helm and Fire and Blood published in its entirety, it’s a good bet the show and book will align quite nicely.