The Lost Boys is one of the early side missions you'll come across after you visit Zalissya in Stalker 2. During the Stalker Wisdom side mission, you'll speak with Lens, the technician, and be able to undertake another side mission where he'll ask you to find some diggers and an NPC named Mastiff that have gone off radar. 

However, during the quest, you'll find that the situation isn’t as simple as it seemed when one of the men tells you a Monolithian—a brainwashed soldier—has taken Mastiff. Should you trust the Monolithian, who is claiming he saved Mastiff, or should you kill him then and there? Here's what can happen and which option is the best one. 

Killing The Ninth

After you meet Max Saturday by the anomaly, he'll explain to you that Mastiff has been taken by a Monolithian, named The Ninth, who killed everyone else. When you visit The Ninth’s location, if you kill him straight away, you'll lose access to the Rubber Layer blueprint which can upgrade your armor, since this is a reward for siding with The Ninth.

The same happens if The Ninth is killed by Dew’s men or you join Dew in the attack on The Ninth after they show up looking for him. When returning to Lens, he will give you the full reward for completing the quest, since he has a distaste for Monolithians. 

Besides that, you can loot the house and grab anything else you might need after The Ninth is dead.

Siding with The Ninth

If you choose to ally with The Ninth by believing him and fighting Dew’s gang, you'll get the Rubber Layer blueprint, which is highly valuable and worth having. You can use this at any technician to upgrade your armor. However, Lens will only give you half of the reward money for completing his quest, due to his hatred for Monolithians. 

He will also issue a stark warning, telling you that Monolithians are monsters and that the Gaffer, who protects Zalissya, will not approve. While this doesn’t affect your standing right now, favouring Monolithians throughout the game can affect how other factions and settlements react to you. 

Should you side with the Ninth?

Overall, it's worth it to side with The Ninth. The blueprint you get is a better reward than the money, and you can easily make up the difference by exploring for 10 or 15 minutes and selling what you find. 

Just be sure to save before interacting with Dew, so that in case The Ninth dies in the fight, you can restart and kill Dew’s men before he perishes.