I just played my first campaign game over the weekend and I'm having a lot of trouble understanding the mission deck and when/how to move on to the next mission.

I was the Imperial player, with two rebel players (each playing two heroes). We played the introductory mission, "Aftermath." The rebels lost. In the campaign guide it says that if the rebels lose, you put the story mission "Under Siege" into play. Here is where my confusion begins.

I understand the basics of making the mission deck. On the campaign tracker, after the Aftermath mission, it states that the next mission is a side mission. But does that trump a mission triggered by the end of another mission? In other words, where do we go next? To "Under Siege," or do the rebel players draw two missions from the deck and select one? In the latter case, when do we then move on to "Under Siege"? Thanks for reading.