Each mission includes a number of grammar and vocabulary lessons necessary to achieve the objectives of that mission. It is possible to find the same lessons in different missions when the lessons are relevant to achieving the objectives in question. You can repeat the lesson to refresh your memory or skip it. In any case, when you complete a lesson, it will remain completed regardless of the mission. Since you can follow the missions in an order different to that proposed in your learning path, each mission can be completed independently and still make sense.