Unlocking the quest
This task can be unlocked by reading a specific announcement from one of the boards in Toussaint, but you can as well move to the place where the quest givers are. It is a small cave located in the western part of the duchy, north-west from Beauclair palace (M2,8). Once you reach the place, try to enter the cave, and a scene with Lucien and August will start. They will ask Geralt to free their brother, Hugo, who stayed in the monsters den. You can begin the quest immediately or first negotiate the payment.
Enter the den. Shortly after arriving to the first bigger cave, Geralt will be attacked by Kikimore Worker and Kikimore Warrior. Try to move around much. To clear the cave, Geralt can use the oil against insects, White honey or Igni sign. Once you win the battle, return to the exploration of the cave, using Witcher senses. Thanks to them, you will notice a sword and blood marks. Use them in further navigation. For a moment, ignore the tank with boiling water, and turn right. You will find Hugo in the next cave.
Fishing out the filter from the lake
Talk with Hugo to hear his side of the story. Hugo will ask for Geralt's help in finding a filter, lost in the cave. Once you end the conversation, return to the lake with boiling water. Use Quen and then jump to the water. Dive and try to locate the Still filter , shown on the screen. You need to find the item fast, because once Quen stops working, Geralt will start losing health points. Return with the filter to Hugo. You'll receive 100 crowns and you will travel in the direction of the entrance.
Once you reach the entrance, a new conversation with Hugo will start, about the arrival of additional mercenaries outside of the cave. You can proceed in two ways:
Decide to attack Auguste, Lucien and accompanying hunters (first dialogue option) A battle will begin, in which Geralt has to beat all enemies. The brothers will be automatically stunned, and after the victorious battle, Hugo will safely leave this place.
Try to convince Hugo, Auguste and Lucien to stop the hostilities (second dialogue option) The attempt will be successful, and you'll receive the prearranged payment.