Diamond City Blues

Diamond City Blues is a side quest in Fallout 4.

Detailed walkthrough
The quest starts in Diamond City where the Sole Survivor must help Paul Pembroke deal with Henry Cooke, the barkeeper of the Colonial Taphouse, and then intercept a drug deal in order to gain access to a chem lab.

The quest is obtained by witnessing Pembroke get in a fight with Cooke over Pembroke's wife. The fight happens when the player character enters the Colonial Taphouse in Diamond City for the first time. Shortly after the scene, Paul Pembroke will seek out the player character in Diamond City, and the actual quest starts.


 * 1) Visit the Colonial Taphouse and watch Paul Pembroke get beaten by Henry Cooke.
 * 2) Wait until later and talk to Paul around town to get the quest.
 * 3) (Optional) Demand payment with a speech check.
 * 4) (Optional) Make Paul stay behind and confront Cooke alone.
 * 5) Follow Paul (or go alone) to the Colonial Taphouse to confront Cooke.
 * 6) At this point the quest takes one of a few different turns, which will determine who is present when ambushing an upcoming chem deal:

However the bar scene plays out, one can either end up going to the chem deal solo, with Cooke, with Pembroke or with both.

Option 1: Confront Cooke with Paul
Paul pulls a gun and things escalate quickly. Either calm him down with one of three easy Charisma checks or encourage him to attack Cooke. Taking the former ensures that both Pembroke and Cooke will be part of the upcoming chem deal ambush, as Cooke tells both Paul and the Sole Survivor directly about it. Taking the latter leads to Cooke's death. The Sole Survivor can get Cooke's note from his body in lieu of hearing about the chem deal directly from him. After reading the note, it is possible to either discuss the deal with Pembroke and enlist him as a backup, or to cut him out entirely by not mentioning it.

Option 2: Kill Paul in the bar
If the player claims they're not with Paul, he will say "thanks for nothing" and one will not be able to persuade him when he draws his weapon. He and Cooke will open fire on one another, but neither will be hostile, presenting an opportunity to side with one by firing on the other. If one decides to help Cooke (or let him kill Pembroke, very likely to happen as he has a stronger weapon and is a higher level), the quest proceeds with Cooke telling the Sole Survivor about the deal. If Cooke dies, the quest reverts to option 1.

Option 3: Confront Cooke alone
This requires passing a medium Charisma check during the initial conversation with Paul. There is a choice between a medium Charisma check and two easy Charisma checks to get Cooke to stop seeing Darcy. If successful, he will no longer allow her in the bar and will tell the Sole Survivor about the chem deal. If failed, or choosing the non-Charisma dialogue, he will turn hostile. Whether one kills Cooke and reads the note or keeps him alive, it is still possible to include Pembroke in the ambush by telling him about it when reporting back to him.

Ambushing the deal

 * 1) Go north to the quest marker, just outside of Back Street Apparel. If one has a companion that is not Strong, X6-88, Cait or Dogmeat they must converse with Nelson Latimer and Trish before attacking them in any way. They are not considered hostile until combat begins, so any harmful action against any of them will be seen as "murdering a non-hostile" by other companions (and counted as murder in the Pip-Boy's crime statistics). This is risky on higher difficulties as the five hostiles immediately open fire from five directions, with little chance to get behind cover.
 * 2) Once everyone else is dead, the Sole Survivor can interrogate Trish. There are a few Speech checks the Sole Survivor must pass. She will also hand over the security password, which is needed to access the chem lab's tripwire terminal. If the Sole Survivor doesn't let her go or does not pass the speech checks, they will not receive Trish's note and won't know how to enter the chem lab. Alternatively, the Sole Survivor can kill her and take it off her body.
 * 3) The outcome of the ambush changes depending on who is still alive after the attack:
 * If Cooke is present at the ambush and still alive, he will attack Trish regardless of what is decided, even if one persuades her to give up the password. It is possible, however unlikely, that Trish will gain the upper hand in the fight (by retreating far enough away that his shotgun will miss and/or picking up a better weapon from a dead Triggerman) and kill Cooke.
 * If Paul is present at the ambush, the player character will need to decide how to split the goods. If one agrees to a 50-50 split, he will take all the chems and set himself up as the new owner of the Colonial Taphouse, effectively replacing Cooke as its merchant. A 70-30 split via an easy Persuade check, or taking everything (and sending him home without him turning hostile, which requires a hard Charisma check), will result in him staying in his original residence, and he will speak negatively to the Sole Survivor from this point on. One can also kill him and take everything, but Cooke will become hostile if he is alive and hasn't moved far enough away.

Afterward, Cooke will either be dead or leave Diamond City for good, thus he will no longer be available as a merchant. The quest continues, alone, to Marowski's chem lab.

Chem lab

 * Head to Four Leaf fishpacking plant on the east coast, being wary of the feral ghouls as the Sole Survivor approaches and of the tripwires around the building. Climb to the top and head towards the terminal on the side to gain access to the lab. If one does not have rank 3 of the Hacker perk, they can access the chem lab by triggering the tripwires in the order depicted in Trish's note.
 * Once the Sole Survivor gains access the quest is considered complete.

Rewards

 * 800-1000 caps looted from Nelson Latimer, if killed.
 * Chems with a nominal value of 9,750 caps:
 * Up to 60 each of Buffout, Jet, Psycho and Mentats, if not splitting 50-50 with Paul.
 * If Nelson is left alive, 25 of each (if split 50-50) or 35 of each (if split 70-30).

Companion reactions

 * If MacCready is present while Trish is killed, it will cause him to leave or turn hostile unless his relationship with the Sole Survivor is already maxed.
 * If the Sole Survivor starts shooting at the drug deal without talking first or kills Cooke afterward, Curie will hate it and leave.
 * Strong's dialogue contradicts his reaction if taking all the chems for oneself, as he says the Sole Survivor should share their spoils in a disappointing tone, even though he likes taking it all.