Though Geralt avoided politics like the plague, there were times when he simply couldn't avoid getting entangled. This was very much the case when Roche and Dijkstra convinced him to find their co-conspirator. The man had gone off undercover to resolve a certain matter among the Nilfgaardians and simply disappeared.
Upon finding Dijkstra mingling among the fugitive mages, then watching him assist them in their flight, Geralt got the sinking feeling the erstwhile spy would want a favor in return. He could not have been more correct, for as soon as the mages had set sail for Kovir's hospitable shores, Dijkstra confessed that he had set in motion a plot to assassinate Radovid and asked the witcher to meet him promptly at the Passiflora.
Meet Dijkstra at the Passiflora
Follow your map to the Passiflora. It is just east of St. Gregory's Bridge. When you go inside, speak to Marquise Serenity and she will open a secret room for you. Head up the stairs and then speak to Dijkstra. Shortly after, Vernon Roche joins the two and Geralt can agree to help with tracking down the spies' missing contact.
Talk to the Redanian liaison
Meet with Gregor, the disillusioned Redanian soldier pointed out by Dijkstra. The easiest way to reach him is to fast travel from St. Gregory's Bridge to the Border Post and then walk a short distance over the bridge. He will direct you towards your next target - a shoemaker's wagon recently found abandoned nearby.
Find the shoemaker's wagon
The wagon is located southeast of the Border Post, southwest of Hindhold and north of Hanged Man's Tree, just east of the Person(s) in distress marker. After finding it, you will have to use Witcher Senses to investigate the area, but not before dispatching some low-level wolves and/or wargs. Follow the trail of shoes southwest to find a rock troll, Rogg, standing over a pile of shoes.
Talk to the troll
Through the conversation with Rogg the troll you learn that he and his friends Ogg and Pogg have the shoemaker unharmed in their custody in a cave nearby.
Find the trolls' cave
The three trolls' hideout is just a few steps away. Enter the cave and pick up the Superior ursine boots, gauntlets, boots and armor diagrams from the small room on the left along the main passage, if you haven't done it in the past. Continue deeper into the cave and find Dijkstra's contact with the other two trolls. You can either threaten to kill the trolls to release him, which turns them hostile, or choose the second dialogue option which will reveal the possibility to have the contact released peacefully.