- Go to Valentine's Detective Agency
- Talk to Ellie Perkins
- Sit down
- Tell Nick Valentine your story
- Follow Nick
- Get the key to Kellogg's house
- Investigate Kellogg's house
- Follow Nick
- Show Dogmeat the cigar
Once you have completed Unlikely Valentine, you will be told to meet up with Nick in his office. Fast travel to Diamond City Market and head straight to his office. On Nick's invitation, sit down and go through the dialogue options to build up Nick's suspicions about a local mecenary named Kellogg who has a history of abducting children. Nick will tell you that Kellogg used to have a house in Diamond City and suggests the two of you head up there to investigate.
Follow Nick up to Kellogg's house. When you get there, the door is locked and locked tight. You can pick the lock if you are a master at lock picking, otherwise you'll need to procure the key from the Mayor's office. Head to the otherside of Diamond City market and ride the elevator up to the Mayor's office. As you walk in, Piper will be arguing with the receptionist, Geneva. You can talk to them, but you'll need to just head through the double doors and into the Mayor's office. Talk to Mayor McDonough and ask him about the key. If your charisma is high enough you can charm him into giving you the key. If that doesn't work you'll be stuck as the Mayor cannot be bribed. You can, however, bribe Geneva for 250 caps. The key is in her desk, so you can either bribe her, charm her or steal from the desk (but risk losing Karma as a result).
However you acquire it, once you have the key head back to Kellogg's house and make your way in. The house has little in the way of items, initially, but look under the desk to find a button that opens a secret door. In here you'll find huge amounts of food and ammo as well as a handful of Kellogg's half smoked cigars. Once you've picked them up, talk to Nick and you'll suggest that Dogmeat could track the scent. Head out of the house and Dogmeat will be waiting outside for you. Show him the cigar and the Reunions quest will trigger.
At this point, you can also elect to have Nick be your companion, which is entirely up to you.
Getting a Clue Walkthrough
Getting a Clue