House of the Dragon surprised viewers with an epic Targaryen reunion between brothers and former enemies, Aegon II (Tom Glynn-Carney) and Aemond (Ewan Mitchell). There's no mention of such a reunion in George R.R. Martin's Fire & Blood, and there's no doubt that it changes everything heading into House of the Dragon season 4, particularly for Aegon.
In the third season, we watched Aegon flee the Red Keep with Larys (Matthew Needham) and make it all the way to Rook's Rest, where he crosses paths with Tyland Lannister (Jefferson Hall). While Rhaenyra's forces search for Tyland, they come across Aegon, who is sick of running. When he makes his big stand in the forest near Rook's Rest, his dragon, Sunfyre, returns from the dead, basically, to save Aegon's life.
Then, of course, Aegon's next move is to fly to Harrenhal, where he finds Aemond, his brother, whom he swore he was going to murder if he had the chance. Suprisingly, Aegon doesn't kill his brother in the season 3 finale. Instead, he demands Aemond join the cause to defeat Ormund Hightower (James Norton) and Daeron Targaryen (Benjamin Evan Ainsworth) before taking back the Iron Throne from their half-sister Rhaenyra (Emma D'Arcy).
Well, things play out a little bit differently for Aegon in Fire & Blood. There are some spoilers from Martin's book concerning Aegon, Rhaenyra, and other key characters ahead. You have been warned.

Aegon's story is very different in Fire & Blood
Yes, Aegon is whisked away from the Red Keep before Rhaenyra attacks. He bounces around for a bit before he hides at Dragonstone, the place Rhaenyra leaves from to take King's Landing. There, Aegon just chills, rests, and tries to numb the pain of his horrific injuries and ailments.
Aegon hides out with the Two Toms, a father-son duo of Tom Tanglebeard and Tom Tangletongue, while the war continues and drags on. Eventually, Aegon and Larys wait long enough for Rhaenyra to lose her grip and power. Then, when the moment is right, they rise up and take Dragonstone with the help of Aegon's dragon, Sunfyre, who finds him on Dragonstone.
It's unclear if Sunfyre knew Aegon was there or if the dragon was returning to the place it was born. Anyway, Aegon uses his advantage to take the castle in secret. Only Baela Targaryen (Bethany Antonia) and Moondancer try to stop Aegon, and there's a big fight between Aegon and Sunfyre and Baela and Moonancer.
Aegon and Sunfyre win that duel, and Aegon officially takes over Dragonstone. Then, he waits, and that wait pays off in spades. Later, Rhaenyra arrives home after fleeing King's Landing to find her enemy waiting for her.

House of the Dragon season 4 will have to change part of Aegon's story
Heading into House of the Dragon season 4, Aegon was last seen at Harrenhal with Aemond, but Rhaenyra already knows that Aegon and Sunfyre live. That is going to make it incredibly difficult for Ryan Condal and the writers to pull off Aegon's big surprise at Dragonstone in season 4.
In Fire & Blood, very few people know Aegon is still alive, and even fewer know where he is. It makes sense for Aegon to stay hidden to keep himself alive. If Rhaenyra's people found him, he's dead.
Then, he makes a calculated risk to take Dragonstone, but even after he does it, he ensures that the secret stays on the island. What happens on Dragonstone stays on Dragonstone.
In House of the Dragon, though, everyone believes Aegon is alive. His supporters are sending ravens around telling everyone that he's been resurrected. Plus, there's still a war going on. I know that there's a lot of confusion and that Aegon is unable to really fight with his injuries, but it is hard for me to believe that Aegon will just let the war play out when he still has Aemond and Vhagar — kind of! — at his disposal.
With King's Landing and likely the rest of the Seven Kingdoms officially fed up with Rhaenyra after her latest blunders at the Great Sept of Baelor and at Tumbleton, Aegon would be more right than ever before to come forward. If that happens and Aegon steps back into the spotlight, there's no way that he makes it to Dragonstone in secret.

There's still a way to pull off the biggest blindside in House of the Dragon history
The biggest blindside in House of the Dragon history is still on the table in season 4, but Aegon needs to be in the right place for it to happen the way it needs to happen.
Of course, we don't know exactly what's in store in House of the Dragon season 4 for Aegon, but there is a way that he could resist the temptation to return too soon. Aegon is at Harrenhal now, and he's with Aemond.
What did Aemond just tell Alicent (Olivia Cooke) that he was going to do? He revealed that his grand plan was to make Harrenhal his home with Alys Rivers (Gayle Rankin) and wait for Rhaneyra to lose control. Then, he would be waiting to step in.
Well, assuming Aemond shares that plan with Aegon, we could see Aegon go back underground, make his way to Dragonstone, and surprise Rhaenyra at the end of this story once a few other big events happen.
Is it likely? I don't know. I would suspect Aegon would be at the center of the action in season 4. I could see Aegon and Aemond heading off to Tumbleton, as it seemed like they were going to do in the finale.
The point I'm trying to make is that Aegon is not where he is supposed to be at this point in the story, and the writers will have to make some important choices quickly to get Aegon to the right place on the board for the epic surprise to be pulled off.
After watching the finale, I understand why we needed Aegon to meet with Aemond again and show that growth as a character. All season, Aegon's one goal was to stay alive long enough to kill Aemond. He has a very different goal when we leave him in season 3.
Knowing what comes next in Fire & Blood and knowing how satisfying that story is, I am quite shocked that Condal and the writers are playing it this way. If Aegon doesn't get to where he is supposed to be, fans are probably going to be quite disappointed. It will feel like we're robbed of one of an all-time blindside and one of the great character deaths in the history of Game of Thrones and House of the Dragon.
I hope Condal and the writers pull this off and find a way to get Aegon to Dragonstone in House of the Dragon, but there's no doubt that his path there already looks very different than it did in Fire & Blood.
