Story related.
This pops after, as Ivo, you free Critter and Nate.
As the Oracle advised, you'll represent Nate in court and should choose the "controlled by a higher power" plea. After that, the game plays out linearly. You'll win the case, get the key, and then have to use the key on Nate's pillory.
Once to get on the Mary and sail off, this achievement should pop.