STORY 05 - Getting A Clue
Objective: Go to Valentine's Detective Agency
After rescuing Nick Valentine from Skinny Malone in the previous story mission, you'll need to meet him in his office in Diamond City. As you walk through the doors of the agency, you'll watch Nick reunite with Ellie.
Objective: Talk to Ellie
During the exchange, Ellie will turn to you and hand over a reward for your efforts in saving Nick. The reward amount may vary depending on if you persuaded her to pay you when you first asked her about the missing detective.
Reward: 100 bottle caps, worn fedora, faded trench coat
Note: You'll also gain access to Nick's unsolved cases (found as files in his office) which you can pursue as sidequests.
Objective: Sit down
Follow Nick over to his desk and sit in the unoccupied chair in front of it.
Objective: Tell Nick your story
There is no right or wrong answers you can give Nick here as you need to exhaust most of the dialogue options anyway before Nick can work out a potential suspect. When you're done giving him the details, he will mention that a mercenary matching your description lived in town until recently.
Objective: Follow Nick
After working this out, Nick suggests you both go and investigate the empty house together. Trail after Nick until he leads you to the front door of a small house after explaining a bit about Kellog on the way. When you arrive, Nick will attempt to pick the lock with no success before letting you try your luck. If you have master level lock-picking skills, you can simply open up the lock and walk on in. If not, you'll need to try your hand at other more devious methods to get inside.
(Optional) Objective: Get the key to Kellog's house
You now have two basic option, get the key from the mayor. Or get the key from his secretary. Either way you'll need to ride the elevator up to the mayor's office to do either. When you arrive, Piper will be deep in discussion with Geneva the secretary. You can join in or just walk past them to chat with the mayor. You'll need to pass a charisma check here for him to hand over the key. Bribery unfortunately won't work on the mayor, but hold that thought. Leave the office and you'll be able to try your luck with Geneva instead. You can persuade her, steal the key or pay her 250 caps for her to 'accidentally' leave it out. Office workers these days are so forgetful *wink, wink.* Whatever happens, take the key back to Kellog's house and enter.
Objective: Investigate Kellog's house
As you walk in, you'll find the place is fairly spartan in it's furniture, with only a couple of desks and a bed in the place. After a minute or two of searching, Nick will advise you to look for a hidden switch. The switch is located on the underside of the ground floor desk. Pushing the red button will slide aside a section of wall, revealing Kellog's secret stash. Steal everything not nailed down including the cigars on the table. Shortly afterwards, Nick will talk to you about what to do. Suggest using Dogmeat's sense of smell on the cigars to track down Kellog. Nick will also offer to accompany you at this point as well if you want him to go with you.
Objective: Show Dogmeat the cigar
Walk out of Kellog's house to find Dogmeat waiting outside (who's a good psychic doggie?). Interact with him and you'll be off onto the next story quest: Story 06 - Reunions, the final story mission of the first act of the game.