You play from scenario 1 all the way to 25. If a character dies, you move to the next scenario but you can no longer choose that character.
If, in the rescue missions, you rescue a dead character, you can start playing him again in subsequent scenarios. If you ever have less than 6 characters alive to choose from, you lose the campaign.
If, in the rescue missions, you rescue a dead character, you can start playing him again in subsequent scenarios. If you ever have less than 6 characters alive to choose from, you lose the campaign.