Contract: An Elusive Thief is a Side Quest in Novigrad. The quest can be started at the quest board on Hierarch Square in Novigrad. From the quest board you must pick up the note that says "Contract: Imp". The "An Elusive Thief" side quest will then start automatically.

Go to the market square at day and talk to the merchant. Now follow the red footsteps on the ground by using your witcher sense. You will reach a house where you must pick up a letter and read it in your inventory. Follow the trail of clues again and you will meet the Doppler. Defeat him in combat and you can choose to either kill him or let him go.

If you let him go, he will give you a lot of coins and a great sword diagram. Return to the market square again to talk to the merchant and end the quest.

Doing this will also unlock The Doppler Effect trophy or achievement.

However, this is also your only chance to in the game to get a Doppler Mutagen, which is required for the Doppler Decoction. If you don't kill the Doppler, you will not be able to make it. Your choice.