Meet Andern in the Sourcestone Market on the east side of the cafe that belongs to the Defiant. You have to talk nice first if you want any information. He sends you to the Public House to talk to some of the Companions.
At the Public House, talk to the first Companion on the left as you enter. Be nice to her and you will learn what you need to know.
Search the garbage piles around town, starting with the one just outside the door on the right. If any street thug sees you they will attack and you will be KoS to his gang for 5 minutes. You need 5 of these notes to make any sense of the street chatter.
The notes lead to Edward Macklen, and he gives up the leader of the conspiracy... a Guardian named Grant Relimar!