Diamond City Blues

Diamond City Blues is a quest in Fallout 4.

Quick walkthrough

 * 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) Follow Paul to the Colonial Taphouse to confront Cooke.
 * 5) At this point the Sole Survivor has the option to do an easy speech check to get Paul to calm down, or encourage him to attack Cooke.

Option 1: Kill Cooke

 * 1) Encourage Pembroke to kill Cooke, then finish the fight. The Sole Survivor can get Cooke's note from his body telling the Sole Survivor about a drug deal going down north of town.
 * 2) Go north to quest marker, don't bother talking to them as all options lead to combat.
 * 3) Once everyone else is dead, the Sole Survivor can interrogate Trish, there's a few speech checks the Sole Survivor can make for extra XP, but it's not hard to get her to give up the details on the chem lab. Alternately can kill her and take a note off her body.  Make sure to grab the chems before leaving.
 * 4) Head to Four Leaf fishpacking plant on the East coast, being wary of the ghouls as the Sole Survivor approachs, and of the tripwires around the building. Climb to the top and make the Sole Survivor's way to the terminal on the side to gain access to the lab.
 * 5) Once the Sole Survivor gains access the quest is considered complete.

Option 2: Don't kill Cooke

 * 1) Discourage Pembroke from killing Cooke, this results in talking about the chem deal and offering to help ambush it.
 * 2) Go north to quest marker, meet up with Cooke and Pembroke, then circle around to catch the dealers in a crossfire. Don't bother talking to them as all options lead to combat.
 * 3) Once everyone else is dead the Sole Survivor can interrogate Trish, there's a few speech checks the Sole Survivor can make for extra XP, but it's not hard to get her to give up the details on the chem lab. Alternately you can kill her and take a note off her body.  Make sure to grab the chems before leaving. If the Sole Survivor let her go Cooke will kill her, Cooke leaves once that's done.
 * 4) Once the fight is done the Sole Survivor needs to split the goods, the Sole Survivor can split it 50-50, pass an easy persuasion check for 70-30, or a difficult one to make Pembroke walk away empty-handed.
 * 5) Head to Four Leaf fishpacking plant on the East coast, being wary of the ghouls as the Sole Survivor approaches, and of the tripwires around the building. Climb to the top and make the way to the terminal on the side to gain access to the lab. Alternately, take another look at Trish's note and pass through the tripwires in the order described in the note.
 * 6) Once the Sole Survivor gains access the quest is considered complete.

Detailed walkthrough
The quest starts in Diamond City where the player character 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.