In Assassin's Creed Rebellion you can perform a variety of missions. They usually consist in getting a letter or killing an opponent. The mission is completed when heroes complete their objectives.
The mission will be considered unsuccessful if all characters involved in the mission are killed.
You can repeat an unsuccessful mission at any time. If you have more characters available at the headquarters, you can simply change your team and restart your mission. However, if you don't have so many heroes in your team, you have to heal them or wait for their health to recover.
However, the best thing to do is to take care of your heroes before resuming the mission. You can make new weapons for them, create new equipment, or simply level them up.
If one or two teammates die, the number of stars will be affected. This is the only penalty for the death of one of the teammates. This will not affect the number of raw materials or rewards.