The quest starts as a new Contract: The Beast of Honorton. You will receive it in Crow's Perch, it will hang as a note on the notice board (M3,9).
Now go to the small settlement of houses south from the Perch. Three alghouls (level 24) can be found between the houses. There you will learn that your questgiver is dead and, after examining the body, the contract will change into the Where the Cat and Wolf Play quest. You must search the houses. You won't find anything interesting inside - your target is the barn. There you will find a corpse near which a cutscene will play. In the building you will also find an alghoul (level 25), two additional ones might come as well.
Once the cutscene ends, leave the barn and pick up the doll found at the entrance with the witcher senses. Then, follow the trail of the small girl. Once you find her, you can calm her with Axii sign or give her the doll. Then, go to the place pointed by her.
There you will find some tracks that will lead you to witcher Gaetan. You can attack him immediately or listen to his version of the events and then make the choice. If you decide to fight, then watch out: he will ask you to drink a potion, and then he will blind you with a bomb. The fight is quite hard, you should use parrying and your favorite signs (all of them are effective). You should avoid using combinations of attacks - the enemy witcher will use a magic shield against it and you can get easily harmed. In that case you should use dimetrium bomb. Once you kill the enemy, you can pick up his sword (low quality, not worth too much).
However, if you let him go, not only will you avoid the battle, but you will receive a new quest as well: Take What You Want. Gaetan will tell you about the location of a hideout from which you can take whatever you want. No matter the decision you made about the witcher, go back to the girl in the village. During a cutscene you will take her to her aunt, to whom you can give 40 crowns as a kind gift (you won't receive anything in return). After completing the quest you will only receive experience points.