Reunions

Reunions is both a quest and achievement/trophy in Fallout 4.

Synopsis
Without Dogmeat fresh on Kellogg's trail, the Sole Survivor will need to track down the kidnapper, but not before overcoming a few obstacles.

Walkthrough

 * 1) As soon as Dogmeat is given Kellogg's scent from the still smoldering cigar, he will immediately lead the Sole Survivor back out of Diamond City, where he will begin heading towards a small pond to the southwest. On a ruined deck near this pond, yet another smoldering cigar will be found, which will need to be given to Dogmeat in order to pick up a more fresh trail.
 * 2) Dogmeat will then lead the Sole Survivor to some nearby railroad tracks, and once reaching the freeway underpass, the party will need to deal with a group of wild dogs in order to pick up some bandaging that contains a trace amount of Kellogg's blood, leftover from a recent hostile encounter.
 * 3) The scent from the blood will lead the party to a metal shack on the same railroad from earlier, which has been heavily booby trapped by an unknown wastelander, which must be traversed in order to discover a bottle of Kellogg's Gwinnett ale.
 * 4) Sniffing the ale, Dogmeat will renew his search, which leads to the party crossing a bridge leading into a decrepit town infested with feral ghouls.
 * 5) After fighting off or sneaking around the feral ghouls, the party will discover a crippled assaultron near the town's exit, which must be spoken to, in which it will then power down permanently.
 * 6) Continuing onward, the party will discover Fort Hagen, where more bloodied bandages can be found, confirming that Kellogg is encamped within the fort's walls.

Fort Hagen

 * 1) The Sole Survivor must enter the fort, either through a roof hatch (which is guarded by a myriad of turrets), or through an unguarded door in the underground parking area.
 * 2) The Sole Survivor will discover that the entire fort has been taken over by a group of synths, largely confirming that Kellogg is working in some way with the Institute.
 * 3) After the generation 1 synths are dealt with, the Sole Survivor travels deeper into the fort, down by use of an elevator, and eventually into the command center.
 * 4) It is within this command center that Kellogg will finally be confronted, in which a short exchange will occur between him and the Sole Survivor.
 * 5) No matter the dialogue choices, Kellogg will become hostile, and he and his synth bodyguards must be dealt with in order to leave Fort Hagen alive.
 * 6) After killing Kellogg and his guards, Kellogg's terminal password and his enhancements must be looted from his body, as the password will need to be used in order to access a nearby terminal and unlock the mag-locked doors leading out of the fort, and the brain enhancement specifically is needed to progress into the next segment of the quest line.