The first Witcher 2 Assassins of Kings Enhanced Edition act begins as soon as you manage to escape The Dungeons of the La Valettes in the game’s Prologue.
Roche agrees to help you after the interrogation and along with Triss and the Commander of the Temerian Special Forces; you sail towards Flotsam looking for the kingslayer, the one who assassinated King Foltest.
The introductory cutscene shows Foltests assassin in a cave talking to Iorveth, and planning his attack on Foltest.
We can also see that the assassin is the same person who attacked the king during the cutscene featured at the beginning of the game.
During the attack of the black dragon (Trial By Fire main-quest), we lost the silver sword we use to kill monsters.
If you open you quest-log you will see that a new side-quest called A Sword for Monsters was added to you journal.
Additionally, your side-quest named The Assassins of Kings was updated.
As we approach Flotsam, we discover that the harbor is closed because a new monster roams nearby.
This means, that we have to reach Flotsam on foot. The first main quest of the first act, A Rough Landing, begins now.