The first task in this quest is to find the child who shines Dettlaff's shoes from the vision. Head to Beauclair Port to complete that step of the quest. Arrive during the daytime to find the shoeshiner being harassed by some unhappy patrons. Defeat them in hand-to-hand combat to move forward with the investigation.
The shoeshiner will strongarm Geralt for information about Dettlaff. He'll want 500 crowns for the info. You can influence the boy to tell you the information without having to spend the coin if you have the Axii Sign leveled high enough. If not, you'll have to negotiate. We were able to get him down to 425 crowns after he rejected our first offer of 350. Regis then comes into the scene to get more information out of the boy.
After the conversation, follow Regis to the house the boy told you of. It's a toy store. Head upstairs to the attic where you'll find a suspicious letter. Read it to trigger a cutscene. After the conversation, Geralt will want to further investigate the room. Find letters of the targets in the left corner of the room opposite to the stairs. The cutscene will complete the quest.