Getting a Clue

Getting a Clue is a main story quest in Fallout 4.

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 character (if they have reached Level 18 and unlocked the third rank of the Lockpicking perk) or a companion such as Cait, or it 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, there is a button underneath the desk which will open a secret room which Nick suggests searching for clues. After the Sole Survivor picks up the San Francisco Sunlights, Nick suggests using Dogmeat to track Kellogg's scent from the cigar. Meeting outside, Nick becomes available as a companion and offers to accompany on this mission. Interacting with Dogmeat to give him the scent completes this quest and starts Reunions.