Story: Season 1

Mission Mastery
Every chapter has 5 stars of mastery, each star obtained grants a different reward:

Fully mastering a Chapter grants players Rewards.

Mission Difficulty
Each mission, save the Premium Missions, has a set difficulty level. When a mission is 5-starred, the difficulty is scaled to the agent level the player is.

In Premium Missions, difficulty is set to the agent level, no matter the mastery.

Challenge Mode sets the difficulty to to the agent level +5, whatever the mastery.