The trailer opens with this shot of Corlys Velaryon's fleet and two dragons flying toward King's Landing:
Then, we see Viserys and Rhaenyra arriving somewhere — presumably Driftmark, seat of House Velaryon, so that Viserys can set up the marriage between Rhaenyra and Laenor Velaryon that he mentioned in Episode 4.
We see this man playing a drum, who may be a nod to — or the actual inclusion of — the character Mushroom, who is a court fool and a source for some of the "history" in Fire & Blood, the book the show is based on.
Then, there's more of Corlys' boats.
And then, we see Viserys and Rhaenyra in the throne room dressed for a formal event — likely Rhaenyra's wedding to Laenor, especially considering the Targaryen and Velaryon banners on the wall behind them.
We get a better look at the dragons that were apparently accompanying Corlys' fleet — it seems like they are the Velaryon dragons, Meleys (Rhaenys' dragon) and Seasmoke (Laenor's, who we saw in battle with him in Episode 3).
Here's another look at those dragons. We hear Viserys say in voiceover, "I hope to herald in a second age of dragons."
There's a shot of Rhaenyra looking pensive — probably on the journey to Driftmark, to be offered up to Laenor.
Then, we have Rhaenys telling Corlys "Rhaenyra's succession will be challenged. Knives will come out."
Next, we have the Lannisters — led by Jason, with Tyland behind him — entering what is probably Rhaenyra and Laenor's wedding. Behind them, it looks like the Hightowers, led by Hobert, are also attending.
There's this shot of Ser Criston in Alicent's chamber, which is significant for a couple of reasons — notably, Alicent is wearing her (mother's) Hightower gown here, not the Targaryen red and black that she's been wearing in recent episodes.
We also see Ser Criston removing his sword. There are a few reasons he might do so around Alicent. In private. In her chambers. HMMMM.
Next, we see a shot of a blonde man being beaten up — this looks like it's Joffrey Lonmouth, who we saw very briefly in Episode 3 alongside Laenor — the two of them are very close.
Laenor is also being attacked — it seems as though fighting has broken out at the wedding.
Corlys looks on as chaos breaks loose.
Ser Harwin Strong is seen punching someone.
And Ser Criston Cole is, too.
A distressed Rhaenyra seems to be caught up in the fray.
Meanwhile, we get another shot of Alicent wearing her Hightower dress.
Here's Corlys in front of what looks like his Driftwood Throne at Driftmark.
We see Daemon wearing a hood and standing in what looks like the Vale — which is where his wife Rhea Royce lives. Perhaps, he's paying her a visit after being banished from King's Landing (again)?
There's a shot of Viserys being attended to by maesters, and he is NOT looking well. The infection in his hand seems to have spread up his arm.
Rhaenyra and Ser Criston stand side by side on what looks like a ship — probably heading toward Driftmark.
There's a shot of someone on a horse as the horse rears up — you can see a hooded figure in front of them.
The hooded figure is no doubt Daemon, choosing violence in the Vale.
We see Alicent, still in that Hightower dress, watching as it appears someone (probably Viserys) collapses in the courtyard below.
The trailer closes with Otto talking to Alicent (who is here wearing Targaryen colors, indicating this probably takes place before the above shots of her in the Hightower dress).
Alicent is very distressed as Otto says, "The king will die, and if Rhaenyra succeeds him, war will follow. Prepare Aegon to rule."
What are you most excited to see in Episode 5 of House of the Dragon?
