If you've played any Gothic games in the past, you'll recognize the storyline in Gothic3. For those who haven't however, the gist of the game is simple. Quite some time ago, a man with no name was imprisoned by a king. Through various actions, he escaped and became a hero in past editions of the Gothic series. There's of course a great deal more to the story, but this article is not here to present spoilers. Its good enough to say that the storyline in all of the Gothic games are well done and need to be experienced.
In Gothic 3, the Unnamed Hero arrives in a new area with some friends to find the Orcs have taken over this area and humans are being subjugated. Not willing to just stand by for that, the Unnamed Hero jumps in and frees these enslaved villagers (with some help from a few friends). Once this introduction to the new world is over, you get the option to choose your path. You can choose to fight and overthrow the Orc oppressors throughout the land. You can also choose to join them, aiding in the tyrannical reign of your new Orcish buddies. Or you can choose a unique and different path, something which is at neither end of the extremes.
This unique choice in storyline is what makes Gothic 3 so interesting, since there is now the possibility of choosing to be against humanity rather than for them - or at least choose to be a mercenary for the dark side.