A Deadly Plot - Novigrad
Location: Passiflora (Novigrad) & North of Border Post
Quest Giver: Automatically started after completing Now or Never
Other Requirements: This quest must be complete before The Isle of Mists
This quest begins immediately after the Now or Never Quest ends. During the final dialogue of the Now or Never quest Dijkstra will approach you and speak with you, which is the pretext for this quest. The first thing you will need to do for this quest is meet up with Dijkstra who is found at the Passiflora in Novigrad.
After speaking with Dijkstra and Roche in Novigrad your quest will update and send you to a location in Velen just north of Border Post. Here you will want to speak with Gregor to update your quest. Our next stop is southeast of Border Post - you'll find the wagon we're looking for right along the side of the road (pictured below).
Inspect the area around the wagon to find signs of a scuffle and a shoe. Follow the trail of shoes from the wagon southwest until you reach a rock troll (as shown on my map). Speak with the Rock Troll here to learn that the troll not only has a few friends nearby but also the person you're looking for.
You'll find their cave just a tad bit southeast of where you talked to the Rock Troll standing outside. In addition to finding the shoe maker and two more trolls inside of the cave you'll also be able to collect all of the Superior Ursuine Crafting Diagrams, they're found in a small room just off the central passage before you reach the trolls.
The trolls will refuse to let the shoemaker go at first and you can either kill them (select option 1) or choose the second option which will allow you to negotiate with the trolls peacefully to release him. Note: If you fight and kill the trolls you'll get the rare Rock Troll Mutagens.
Rock Troll Weaknesses: Ogrid Oil, Quen
Upon freeing Thaler escort him back to the wagon for a little bit more dialogue then the quest will complete.