"Lexington and Concord" is a mission in Sequence 7
Rescue the Civilian hostages
Kill a group of regulars with a single order - 0/7
Head for the marker visible on the Frontier. This is on of the longer missions so be ready for it. After this mission, for the next 2 parts, you will be unable to do side quests. Take care of any liberation and recruitment you have been meaning to do now.