The side quest "Following the Thread" in The Witcher 3 is one of a very small number of quests that demand Geralt visit two regions before they finish. It's also one of a few quests that center around a recurring character who has appeared in previous games.
In this case, the star of the show is the fellow School of the Wolf witcher Lambert. Fans may remember him from the first game, from the book series, and also from season 2 of the Netflix series. And because this quest involves Lambert, it's worth noting that "Following the Thread" will fail or become inaccessible if players complete The Witcher 3 main quest "Ugly Baby" first.
Monster in the Bits
This quest begins as an ordinary monster hunt. Geralt will find an open contract on the notice board in Hierarch Square titled "Monster in the Bits," and the map will indicate that the quest giver is due east near the Southern Gate. Unlike a regular contract, however, someone else is also on the case and Geralt won't be able to negotiate his payment.
Instead, Geralt must investigate a nearby storehouse, and by checking the clues he'll determine that the monster is an ekimmara. Ekimmaras are vampires, so players should ready their vampire oil and Black Blood potions. The vampire's trail leads across the river to a water mill, and that's where Geralt will encounter both the ekimmara and Lambert.
Unfinished Business
Lambert will want to leave before Geralt can loot the ekimmara's body, but players can choose to catch up with him later or go back for the loot after the next sequence. Either way, the quest continues with Lambert demanding that the contract giver tell him about Jan Karadin, and when the man runs off Lambert will chase him and Geralt will have to deal with three Redanian guards. Once Geralt catches up, Lambert will kill the contract giver before Geralt can react.
The quest picks back up at the Seven Cats Inn, which is a short distance to the south. Lambert explains why he's going after Karadin and his associates, and once he's done the two head inside to speak to an elf named Vienne. She doesn't know where Karadin is, but she mentions several other names the witchers can track down. Geralt can then ask Lambert to spare Vienne, and if he doesn't he'll have to fight her and three other elves.
Skellige Bound
Geralt and Lambert will split up at this point, and Geralt's next stop is in Skellige. Specifically, he has to visit Faroe Isle to the southeast and look for a pirate named Hammond. To start this hunt, Geralt must head to the isle's eastern settlement, Trottheim. Note that another quest, "Flesh for Sale," will be unavailable if this part of "Following the Thread" is active when Geralt first visits Faroe. If this isn't Geralt's first visit, players should still approach Trottheim from the outside instead of fast-traveling to its sign post so that the quest trigger will fire correctly.
The guards at the gate will refuse to let Geralt through, but they'll tell him Hammond is to the east at a temple. If Geralt mentions he's surprised by Hammond's piety, the guards will realize he's an enemy and the whole pirate town will turn hostile. Either way, once Geralt reaches the temple he'll have a brief conversation with Hammond and then a fight will break out. Fortunately, Geralt will find a letter on Hammond that proves Karadin is in Novigrad. Also, the pirates of Trottheim will be hostile when Geralt returns if they weren't already.
Just Desserts
Geralt will find Lambert at a place called Nowhere Inn in eastern Novigrad. Lambert has had more luck than Geralt and knows exactly where to find Karadin, and if the player chooses the action will jump straight to his home.
Karadin surprises the two witchers by inviting them in and speaking with them in front of his wife and children. He claims to be retired from being a witcher and an assassin and is now a peaceful merchant, but the evidence Lambert and Geralt found indicates that he's also involved in the slave trade. Geralt can choose to either kill Karadin or spare him. Karadin will fight if attacked, but he isn't very dangerous.
Lambert will be angry if Geralt chooses to spare Karadin, but neither ending will change his story in the future. Also, players can speak to Lambert as he's walking away right after the quest ends. This is an early opportunity to challenge him to a game of Gwent if the quest "Gwent: Old Pals" is active.
The Witcher 3 is available now on PC, PS4, PS5, Switch, Xbox One, and Xbox Series X/S.