You get this achievement by completing the secret mission story, “Closing Statement” It is unlocked by doing a few things first.
1. Start “The Tour” mission story, It is started by finding Corvo Black and a guard speaking next to a railing inbetween the tasting room and reception, on level 2.
2. Do not speak to Corvo, instead follow him up the stairs and then push him over a railing and grab his disguise.
3. Follow and complete “The Tour” mission story disguised as Corvo.
4. As soon as “The Tour” ends, the secret mission story “Closing Statement” will start automatically, I won’t give any details about it as it’s a very good mission story, my personal favourite of the entire game.
5. You’ll get the achievement upon completion of this mission story.