The story for Mask of the Betrayer is a direct sequel to the plot of Neverwinter Nights 2, set almost immediately following the events at the main game's end. The main campaign is set in Rashemen near the kingdom of Thay. The Red Wizards of Thay are a driving force behind the campaign story.
For the official expansion campaign, the player may import their character from NWN2 or create a new character, though any character created is assumed to be the same one that finished Neverwinter Nights 2's main campaign, rather than an entirely different persona. If an imported character is less than level 18 at the beginning of the game, the character will automatically be raised to level 18.