This final chapter of the first act calls for a long, long hike and some creature encounters along the way. Thankfully, you have your companion of choice and Dogmeat as a bonus companion. Man's best friend will use his sniffing skills to hunt down Kellogg one clue at a time. When he first stops (at a pond), look for a cigar in the immediate area. After another stretch of walking, Dogmeat will stop again. This time, look for a bloody rag at a freeway underpass. The next time he stops, it will be at a railroad shack. Look for another cigar there. This will then lead to the remains of a robotic Assaultron. Speak to it for the next clue.
This will finally lead you to the quest's main setting, Fort Hagen. At the fort's perimeter, Dogmeat will lead you to one more clue, another bloody rag for you to analyze. You can enter the fort one of two ways, either through an underground parking lot or through a hatch on the roof. Whatever you decide, be ready for a lot of turrets and synths. Long-ranged weapons will be especially useful here since there are a number shootouts in long hallways, sometimes with few cover points.
As you follow the waypoints, take your time exploring, as there are a lot of healing items and ammo for your to pick up. After taking the elevator, take a quick detour into a kitchen adjacent from the main path. Go to the entrance of the kitchen to grab the Energy Weapons Bobblehead. Go back on the main path to continue your journey to Kellogg. Plan for a few more synths and turrets before finally meeting him. Given the muscle he's brought and your character's desperation, you know this is going to be another boss fight. After a brief question and answer session, the fight will commence.
Similar to the fight against Skinny Malone, this is another area where you can use the local surroundings to your advantage, whether it's flanking Kellogg or hiding to catch your breath and heal. The map is long enough that you can split from your companion and run to the other end of room to divide the enemies. With foes trying to chase after you, this fight makes for a prime opportunity to use drop a couple mines.
When dealing with Kellogg directly, he'll be using stealth camouflage at first. He's not completely invisible; you just have to look for distortions in the background to spot him. While having Nick as a partner, we found that the private eye has a keen eye in spotting Kellogg. If you're having a tough time aiming at Kellogg, just aim where Nick is aiming. Once Kellogg's camo wears off, be ready to move often as he'll start to throw a lot of grenades. Land enough hits and he'll eventually die.
You'll need to loot Kellogg's corpse of two items: his terminal password and his Cybernetic Brain Augmenter. Use the nearby terminal to open the security doors to the left of the terminal. Also note there’s another issue of RobCo Fun next to the terminal. It will unlock the Pipfall holotape game. Once you exit the fort, you can fast travel back to Diamond City. Conclude the quest by speaking with Piper at either her newspaper office or at the Valentine Detective Agency.