If you play The Witcher 3: Wild Hunt on the PC, you may import your Witcher 2 save files so that the third game in this role-playing trilogy reflects the major decisions you made in the previous adventure. However, you may not be playing on PC, or perhaps you lost your save files, forgot what happened in the story, or never even played The Witcher games prior to Wild Hunt. The good news: The Witcher 3 allows you to select those decisions as if you played its predecessor. The bad news: If you didn't play The Witcher 2, you may not understand the sequence that presents these choices.
In this Witcher 3 sequence, Nilfgaardian emperor Emhyr var Emreis has invited you to the palace at Vizima. Before you meet the ruler, however, commander Morvran Voorhis subjects you to a shave, during which he asks you some probing questions. If you haven't played The Witcher 2, you aren't likely to know the background to these queries. (You can simply skip this Q&A sequence if these matters don't interest you by selecting not to simulate The Witcher 2 decisions when you start the game, if you prefer.) Fear not, however: I am here to provide some context, so that you don't feel completely lost when Voorhis confronts you--perhaps only slightly lost. I will not be telling you how each of those decisions plays out over the course of The Witcher 3, though some of those possibilities are notable: In The Witcher 3, entire quest lines appear that are only available if you choose/chose a particular path. However, my aim is to give you proper backstory so you understand what these decisions mean, and don't have to make them blindly.
Warning: The Witcher 2 spoilers ahead!
Choice 1: Did you kill Aryan La Valette, or allow him to live?
Aryan La Valette is/was the son of Mary Louisa La Valette, the mistress of King Foltest of Temeria--the king whose assassination is at the center of The Witcher 2's story. In The Witcher 2's prologue, you confront Aryan, who is commanding the guard that provides his castle's defenses. You can kill Aryan in this scene (and his guards, if you so wish), or allow him to live. Killing him opens up interactions with his mother later in The Witcher 2; should you force him to surrender, you encounter him later in The Witcher 2, and might even aid in his escape from the castle.
Choice 2: Did you leave Flotsam with Vernon Roche, or with Iorveth?
Vernon Roche is a loyal servant of Foltest who interrogated Geralt in the dungeons of La Valette castle. However, Roche came to believe that Geralt did not assassinate Foltest, and assisted him in his escape. Iorveth, on the other hand, has no loyalty towards Foltest, or any other human king. Iorveth is an elf and the commander of a Scoia'tael unit. Scoia'tael are non-human (typically elven) freedom fighters/guerrillas driven by their abhorrence of humans, yet known to ally with human leaders should such an alliance prove worthwhile. You associate with both of these characters in the border town of Flotsam, where most of The Witcher 2's second act takes place.
Whether you side with Roche or with Iorveth is The Witcher 2's primary choice, as it has enormous repercussions on the chapters that follow. Siding with Roche and his unit, known as the Blue Stripes, opens up a series of possibilities and quests known as "Roche's Path." You may, for instance, assist Roche in killing Henselt, King of Kaedwen--or refuse to help Roche in such an instance. Traveling down "Iorveth's Path," of course, introduces a different series of quests and decisions, opening the possibility of a friendlier world for Iorveth and his fellow non-humans.
Which choice you make in this dialogue then affects the next choice available.
Choice 3 (Iorveth's Path): Did you save Triss at the Loc Muinne summit, or did you lift the spell that held Saskia?
Oh, Triss Merigold. She is Geralt's primary romantic interest in the Witcher games, and is a sorceress of some renown. Saskia is a beloved warrior who led a revolt when Kaedweni King Henselt tried to invade her land of Aedirn. At this stage of The Witcher 2, Triss is imprisoned in a Nilfgaardian war camp, while Saskia, who is revealed to actually be a shapeshifting dragon, is under control of powerful sorceress Philippa Eilhart. (Sadly, if you take Roche's path, you never discover that Saskia is a dragon.)
Rescuing Triss not only allows you to have some fun in the camp, but it also allows Triss to accuse Síle de Tansarville, another powerful mage, of being the puppetmaster behind Foltest's death. Saskia, however, remains under Eilhart's thrall, leading to a later confrontation between witcher and dragon. If you choose to release Saskia from her curse, Triss is still freed, in this case by Letho, the true kingslayer--yet there are damaging repercussions for mages, who are now the victims of a witch hunt that would not have begun had you freed Triss from her bounds.
Choice 3 (Roche's Path): Did you save Triss from her Nilfgaardian prison, or did you save Anaïs La Valette?
Remember Mary Louisa La Valette, Aryan's mother? Well, she had two children that were fathered not by her husband, but by King Foltest. (Aryan, however, was pure La Valette.) That makes Anais, the older of the two, the true heir to Foltest's throne. She is captured and held by Henselt's forces at a Kaedweni camp, and should you choose to rescue her, you must again make a choice, handing her over either to Redania or to Temeria. As with Iorveth's path, Triss does not die in this scenario; Letho frees her, and she waits for you with the kingslayer in the final scenes.
On the other hand, Triss's blazing red hair is difficult to resist, and should you heed its call, Triss is free to point a finger at sorceress Síle de Tansarville for her murderous politicking once you release Triss from her Nilfgaardian shackles. As you can see, you get an option to assist Triss whether you follow Iorveth's path or Roche's, and the consequences, at least those related directly to Triss, are more or less the same. Anais still lives in this scenario, rescued by Vernon Roche himself, but she will never be treated like royalty.
Choice 4: Did you save Síle de Tansarville from her defective megascope, or did you let her perish?
A megascope is a magical device good for teleporting and having conversations at a distance, among other fantastical uses. Unfortunately for Sile, Letho had sabotaged her megascope. You watch Sile try to teleport away, only to realize the danger and beg for Geralt to help. If you save her, she disappears--but only after giving you a hint as to where you can find long lost love Yennefer. If you let Sile die, well, she become magical mush. Good riddance!
Choice 5: Did you kill Letho, or allow him to walk free?
Letho is a witcher and the man who committed the crime Geralt is accused of. You meet him several times over the course of The Witcher 2, but your final act is to choose whether Letho should live as a potential ally, or die for his crimes. It may be your last decision in The Witcher 2, but its consequences are simple, as the game then ends either way.