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) (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 Sole Survivor has the option to do an easy speech check to get Paul to calm down, or encourage him to attack Cooke. If Paul is left behind there is no speech check, so you can decide yourself.

Option 1: Kill Cooke

 * 1) Encourage Pembroke to kill Cooke (or kill Cooke yourself if you are alone), 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. If you have a companion with you that is not Strong or X6-88, you MUST converse with everyone 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 your companion.
 * 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 Trish's 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 (or decide not to kill Cooke if you are alone), this results in talking about the chem deal and offering to help ambush it.
 * 2) Go north to quest marker, meet up with Cooke (or also Pembroke), then circle around to catch the dealers in a crossfire.  If you have a companion with you that is not Strong or X6-88, you MUST converse with everyone 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 your companion.
 * 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 Trish's 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. If you have confronted Cooke alone you can take all of the goods. Alternatively you can kill Pembroke to take all of the goods once Cooke walks away. The quest will proceed normally to the Four Leaf Fishpacking Plant after this.
 * 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.