A Poet Under Pressure - Novigrad
This quest is automatically given to you upon completion of The Play's the Thing Story Quest. First thing you'll have to do for this quest is meet Zoltan east of the Seven Cats Inn which is in the north-central portion of Velen. Go through the dialogue with Zoltan to trigger a scene with the convoy arriving.
Note: If you got information about Dijkstra's treasure during the Count Reuven's Treasure Story Quest then you'll be able to ask him for help with this ambush. He will give you some extra men, which aren't really neccessary.
A fight will break out when the convoy arrives; rather than fight your job is going to be following the caravan that sprinted away with Dandelion on it. Use your Witcher Senses to uncover the foot prints on the ground and follow them to an area north of where we are now.
The hoof prints will lead you quite a ways north of where we first began, you can either follow the hoof prints the entire way north or cheat and just run up to the Isolated Hut (shown on my map below). Here you'll find a band of dwarves arguing about one lad that just came into their town - the people we're looking for.
Speaking to the dwarves will get you the Key we need to the cellar, after that all that's left is to bust into the cellar and take out the Witch Hunter holding Dandelion captive. Freeing Dandelion will complete this quest and transition into the next Ciri's Story: Breakneck Speed Story Quest. Another secondary quest by the name of Cabaret will be added to your quest log as well once you complete this quest.
Note: Completion of A Poet Under Pressure allows you to begin the main story of the Blood and Wine expansion pack. The quest Envoys, Wineboys is added to your quest log at this time, assuming you have the expansion.