I Work Alone (previously named A Gentleman's Agreement) is one of the achievements available in Monaco. It is earned by completing all missions in Act I in single-player. As of September 2014, only 1.1% of players on Steam had earned the achievement.
Bring the Bleach | Cerveau | Dem Bones | Identified | I Work Alone | Massacre | Play It Again Sam | Silent Withdrawal | Slippery | Technician | Time to Retire | Unscathed | Whacked