It will automatically start through a conversation with Sigismund Dijkstra at the end of Now or Never. Prerequisite: This quest must be completed along with a peaceful solution with Dijkstra at the end of Blindingly Obvious to unlock Reason of State.
Note also you'll need to have at least obtained and completed or failed the other two Assassins' quests, An Eye for an Eye and Redania's Most Wanted. Note: this quest will fail if not completed before starting The Isle of Mists. Follow your map to the Passiflora. It is just east of St. Gregory's Bridge.
When you go inside, speak to Marquise Serenity and she will open a secret room for you. Head up the stairs and then speak to Dijkstra.
Shortly after, Vernon Roche joins the two and Geralt can agree to help with tracking down the spies' missing contact. Meet with Gregor , the disillusioned Redanian soldier pointed out by Dijkstra.
The easiest way to reach him is to fast travel from St. Gregory's Bridge to the Border Post nearby. The wagon is located southeast of the Border Post, southwest of Hindhold and north of Hanged Man's Tree , just east of the Person s in distress marker. Mark the wagon on your map for later, and follow the trail of shoes southwest to find a rock troll , Rogg, standing over a pile of shoes.
