Using Witcher Senses near his stall you'll see he was telling the truth: there's tracks like that of a large cat. Follow them up to the point you reach a wall where you can examine some of the food the imp dropped. The tracks now have also changed to that of scratches.
Follow this new set until you get to the bridge and Geralt will comment that the imp must have dove into the moat. As you might have guessed, jump over and swim towards shore to see the tracks pick up again, but examine them to see they've changed to that of footprints. They lead just a short distance away to the house with the wide open door, so stroll in and pick up the short letter on the table and read it to learn the imp is a doppler, earning 25 . As dopplers can take any form, you'll need a way to track him down.
Head upstairs and examine the clothes to pick up a strong scent of lavender and follow this trail outside where you'll eventually see an Eternal Flame guard talking with an elf in a rather unusual friendly tone. Interact with them and they'll pretend the guard was actually harassing the elf before the elf goes off. Choose whichever dialogue option you want and the doppler will morph before running off. Important: make sure you follow closely. If he gets too far away, the quest will fail.
Once you get close enough to him, hit the interact button to grab him and after a short exchange of words, he'll fight you as a replica of Geralt, complete with abilities. Knock his health down to 1/4 and he'll surrender and ask for mercy. You can then choose to let him go or kill him.
Note: If you try to run away after starting the fight he will indefinitely pursue Geralt everywhere. Even if you manage to fast travel very far away he will run all the way to continue the fight. He will not follow Geralt if you go to another map like Skellige, however, he will continue the chase as soon as you return to the Velen/Novigrad map. So the only way to get rid of him at this point is in fact to defeat him.
If you have completed the main quest Count Reuven's Treasure but not finished the subsequent main quest The Play's The Thing, a third dialogue option will present itself after catching and defeating the doppler. Geralt will ask him to impersonate Caleb Menge and sail to Temple Isle to trigger Dandelion's release. Horrified, the doppler pleads instead to be killed by Geralt than be subjected to torture. Geralt relents and lets him go. This option yields no additional rewards as the doppler runs off without giving a bonus of crowns or a weapon diagram, and the merchant will only pay half the reward as Geralt cannot produce a trophy.
If you let him go, you'll only get half the reward from the merchant but the doppler gives you 25 , 215 , and the Diagram: Gnomish Gwyhyr for letting him go. More importantly though, letting him go means you won't get a doppler mutagen or the trophy. If you choose to kill him, he won't take on the witcher form again, making it a bit easier to kill him (but not earning any rewards here other than the trophy and mutagen).
Whichever decision you chose, go back and talk to Sylvester. If you killed the doppler, he'll be pleased and give you the full reward (245 and 182 ). However, if you let the doppler go, he doesn't appreciate Geralt's remark about having no head to show as proof and will only give half the amount (245 and 91 ). The quest will then complete.
- A Novigrad merchant by the name of Sylvester Amello had hung a notice on the Novigrad board asking for help in tracking down a thieving imp. It seemed the merchants' guild was being pestered by a mysterious creature who kept stealing wares from its members stalls. Geralt accepted the job.
- The thief's trail took Geralt to Novigrad's extramural paupers' district. The thought of venturing into that tough quarter makes me wince. Not so much because of the danger - I have braved much meaner places - but because of the smell, which can be so rank as to bring vomit to one's lips upon strolling within ten paces of it. Geralt, however, braved the worst and soon determined the imp was in fact a doppler, one who changed his form more often than other men change their knickers - although, given the hygienic habits of men in this particular district, that was not saying much.
- The doppler had no choice but to seek salvation in flight. It seemed Geralt was in for a footrace.
- If Geralt spares the doppler:
- Geralt has a soft heart. Though this does not always move him to mercy, this time it led him to spare the thieving doppler - under the condition that the shapeshifter leave the city at once. After that was settled, Geralt talked to the merchant who had issued the contract and brought the matter of the doppler thief to a close.
- If Geralt kills the doppler:
- To this day I think Geralt did wrong in killing that doppler. In my opinion, his crimes did not match the punishment meted out to him - but, in the end, I am no witcher, and so it is not my place to judge. The merchant Sylvester was, unlike me, thrilled with how the witcher resolved the matter and gave him the promised reward without a second thought.
- Go to the marketplace during the day and talk to Sylvester Amello.
- Follow the imp using your Witcher Senses.
- Search the house using your Witcher Senses.
- Read the letter.
- Find a way to track the doppler using your Witcher Senses.
- Follow the scene of perfume using your Witcher Senses.
- Confront the doppler.
- Catch the doppler.
- Defeat the doppler.
- If Geralt decides to kill him:
- Kill the doppler.
- Take a trophy.
- Return to the marketplace during the day to talk to Sylvest Amello.