I posted this in the other thread and thought I'd provide it here, my very brief recap of the two games:
The Witcher 1: Geralt of Rivia, a Witcher (part of an dying order of mutated monster slayers) wakes up without any of his memory outside the stronghold of the Witchers. While he is being taken care of the stronghold is attacked by some mystery cult/crime syndicate and some Witchers die. Afterwards they all agree to investigate the cult and figure out why the fuck they were attacked and split up to do so. LONG, LONG story short, it turns out it was all the masterplan of the "insane" leader of the Order of the Flaming Rose, a highly respected knightly order that often clashes with non-humans (elves, dwarves, etc.). See the leader believed that the end of the world was nigh upon us all and would cover the world in ice and death and something about the Wild Hunt. Geralt kills the dude and chalks it up to the dude being crazy. In doing so he saves the city of Vizima and once again is in the debt and good graces of King Foltest.
The Witcher 2: Assassin of Kings: Geralt now works essentially as a bodyguard for King Foltest, at the end of TW1 the king was almost assassinated by a mystery dude who Geralt believes was a Witcher, though he keeps the info to himself. Foltest is currently waging a civil war to claim his bastard children and Geralt aides him stating that after the battle is done he plans to leave his service. Well, things don't go as planned. They win the battle, but as Foltest is reunited with his children, a big muscular Witcher disguised as a priest assassinates the King and hops out a window. Naturally, Geralt is the prime suspect in the murder and is imprisoned. So begins a long adventure in which Geralt must clear his name and uncover the mystery of the assassins of Kings.
LONG story short, at the end of everything you discover various plots within plots between the various Kings of the Northern Realms and an attempt by the Sorcerers to re-institute The Lodge, basically a magical school and Order that once held powerful sway at Court. But, the real master genius behind it all is the Emperor of Nilfgaard Emhyr (Charles Dance in those trailers you've maybe seen). See it turns out you knew that Witcher who assassinated Foltest and later several other Northern Kings, basically thanks to your shenanigans and what not before your memory loss, he and his band of Witchers got caught in Nilfgaard. The Emperor valuing their skills offered them a mission, go to the Northern Realms and spread chaos through assassination and other means to make the North ripe for invasion. In return the Emperor would restore The Witcher schools in the South (remember they are a dying order). You are told all this in the end by Letho, the big Witcher who assassinated Foltest, and you can choose to fight him or just let him go. At the same time you are told all of this Geralt regains the remaining forgotten parts of his memory and remembers that he was taken by the Wild Hunt, in exchange of the safety of the love of his life Yennefer, leading to his memory loss.
The twist reveal ending is nice because we are trained to always have final physical battle at the end of a game and yet here you can just let the "bad guy" go. Yet, after all the reveals it's not really sure that Letho is a bad guy. Geralt is a Witcher and he has no real loyalty to the Northern Realms or Kings, in fact the game shows you that most of the Kings are complete assholes. Plus, the fact that the Emperor of Nilfgaard seeks to restore the Witcher schools is a benefit to you. It all depends on your view point, no good choice or bad choice. Anywho, at the very end we see a cutscene showing the beginning of Nilfgaard invading the North.