Getting a Clue

Getting a Clue is a main story quest $$

Detailed walkthrough
At Valentine Detective Agency, Nick Valentine will question the Sole Survivor about their missing son. The conversation eventually leads to Nick suspecting a certain notorious mercenary named Conrad Kellogg as the kidnapper. Kellogg was recently seen living in Diamond City with a young boy, but has since disappeared. To confirm his suspicions and search for clues, Nick suggests investigating Kellogg's House. The door is locked with a Master lock, but the door can either be picked by the player (if they have unlocked the third rank of the Locksmith perk) or a companion such as Cait or an Automatron with a lockpick module installed. Alternatively, the door can be unlocked with a copy of Kellogg's house key.

There are six ways to obtain a copy of the key:
 * 1) Persuade Geneva, the mayor's secretary (medium Charisma check, hard Charisma check, or bribe of 250 caps)
 * 2) Persuade Mayor McDonough (medium Charisma check)
 * 3) Pickpocket McDonough
 * 4) Steal from Geneva's desk (Novice lock)
 * 5) Steal from McDonough's safe (Advanced lock, or unlock with Novice terminal)
 * 6) Give chems to Mama Murphy (unlocks an option to search the stadium chairs beneath Kellogg's house, which instantly yields a copy of the key)

Once inside, pressing the button underneath the desk reveals Kellogg's secret room behind a wall, which Nick suggests searching for clues. After the Sole Survivor picks up the San Francisco Sunlights, the duo decides to use Dogmeat to track Kellogg's scent from the cigar. Nick becomes available as a companion and offers to accompany the Sole Survivor on this mission. Interacting with Dogmeat to give him the scent completes this quest and starts Reunions.

Companion reactions
Although X6-88 has one listed reaction, it is impossible to have met him at this stage in the story. Likewise, Paladin Danse is not available as a companion at this point.

Bugs

 * If the office is exited through the second floor door or the player fast-travels as soon as they leave the agency, Nick will only say "Kellogg's house is this way" and will not move.
 * Entering the office again and exiting through the main door, or walking towards the main door will fix this.
 * Completing the quest does not trigger Reunions.