Getting a Clue

Getting a Clue is a main 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 there are a number of ways to get in:
 * The lock can be picked, either by the Sole Survivor or a companion, such as Cait.
 * Mayor McDonough can be convinced to hand over the key to the house.
 * If McDonough isn't persuaded, his secretary Geneva can be convinced to hand over the key.
 * The key can be pickpocket from McDonough.
 * If the appropriate stage of The Sight is completed, Mama Murphy will reveal the location of a key hidden underneath Kellogg's house.

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.