This is not so much a quest as a little trip and a cutscene…
The Witcher 3: An Invitation from Keira Metz
The quest immediately follows the Magic Lamp side quest.
All you have to do here is go and visit Keira at her house and talk to her, and it’s complete. Ooh, she’s a sort, isn’t she?
From here you can go on to A Towerful of Mice.