This quest is available after starting Get Junior after the ambush at the bath house.
You have the option to ask Dijkstra about Dandelion and during the ensuing conversation he will lead you down a secret passage way to where his gold was stolen. Geralt knows it was Dandelion but you do not need to tell Dijkstra that. Use Witcher Sense to search the area especially the broken pipe at the entryway to the sewer, doing that will outline the path you should follow.
GAMER NOTE: You have to use the antidote Dijkstra gives you in order to survide the sewers. The only enemies you'll encounter are Drowners so make sure to use Nechrophage oil before venturing too far into the sewer.
He will ask you to investigate who is responsible for stealing the treasure and Geralt will help him becomes in doing so you will find out what happened to Dandelion. After investigating the secret passage he will ask you to investigate a house where you will end up being reunited with Triss who will help you on the rest of your journey. When you discover that Dijkstra chose both of you because you used to be lovers comment “You Used Us” if you want to be more of a romantic.
This next part is pretty rough, Triss tells you that the only way to get an audience with Menge is for Geralt to pretend to turn her into him risking almost certain torture.
DECISION POINT: INFILTRATING MENGE’S COMPOUND
PATH # 1: The best outcome is to go along with Triss’s plan without initiating combat at all which can seem very difficult since Menge’s men treat her horribly and when you meet Menge you have to listen to her being tortured. DON’T ask anyone not to torture Triss DON’T use Axii on the guards and DON’T be aggressive or mean to the guards. Yes, Triss is being tortured but getting information is more important, go along with it in order to stay in her good graces. Plus, when Menge and Geralt go to check on her she’ll more than makeup for it, promise.
You have to sneak out of the compound as well. If you don’t that will lead you to PATH # 2. there is a locked back door you can exit out of with Triss and she will burn the building to the ground and teleport away leaving Geralt to escape alone. Climb up on the roof of a nearby building and jump the wall, after that there’s a wall you can use Aard on and one of Dijkstra’s men will be waiting for you.
RESULT: Dijkstra remains loyal to Geralt and helps him later on in the game. PATH # 2: Any decision deviating from turning Triss in will lead to combat. You can start combat at the gate, and after climbing the steps outside Menge’s private quarters. If you make it to Menge you can also initiate combat with him by asking him not to torture Triss. If you do this then you’ll have to kill everyone and Triss will be annoyed with you.
Instead of learning about Dandelion’s imprisonment from Menge you have to talk to Dijkstra about a letter you find in Menge’s desk and track down and interrogate a spy who tells you what Menge would have had you not slaughtered him.
RESULT: Dijkstra is angered by Geralt because killing Menge and his men loses him his gold. Dijkstra does not remain loyal to Geralt later on in the game.
In the end no matter what you do it is clear that you will need to speak with Dudu and find a Doppler to impersonate Menge in order to get Dandelion freed. The conclusion of this quest opens up the main quest The Play’s the Thing.