The main game has 13 missions, divided into 3 acts, each with a final Boss. This is intentional, as 13 is the sacred number of the Horned Rat, the Skaven god. After defeating the Boss of each act, you can progress to the Final Mission. Missions can be played on four Difficulties, with the Winds of MagicDLC adding a fifth.
A repeatable Prologue mission is played automatically the first time you start up the game after which you are taken to Taal's Horn Keep, the home base of our Heroes.