To begin this quest, head to the Colonial Taphouse in Diamond City. If you're having trouble finding it , go up the stairs leading to the mayor's office. Be sure to head inside to the bar itself, rather than inquiring at the patio outside. You should find barkeep Henry Cooke, along with Paul and Darcy Pembroke, a married couple in the middle of an argument. Paul isn't fond of Darcy's frequent visits to the bar, and that it leads to a fight with Henry. Paul will eventually storm out of the joint. You can make small talk with Henry and buy Darcy a drink. You can also choose to leave without speaking to them, since exiting the bar is the only way to advance this quest.
This is an emotionally charged situation, one that can have multiple outcomes, depending on who you side with (assuming you side with anyone). To further advance this quest, you can either talk to Paul in the middle of the market or leave Diamond City and then immediately come back. The sad thing is that no matter what you decide, someone will die the moment you return to the Colonial Taphouse.
In our video, Paul dies, making Henry come to terms with his violent ways. He asks you to help him with one last job before he goes clean. He'd like you to ambush a chem deal near Back Street Apparel. A dealer named Nelson isn't too happy about your intrusion, and it doesn't take many words before he and his gang start firing. When you've defeated them, speak to Trish, who you should have wounded by now.
Dead or alive, talking to or searching her will reveal the location of the chem lab. Travel southeast to the Four Leaf Fish Packing Plant. There are Ghouls inside to help you increase XP and gain loot, but you'll need to explore the plant's exterior to complete this quest. Ascend the outside stairs and search the rooftops for a terminal. Also be mindful of the tripwires, which are easy to jump over. Once you reveal the secret door using the terminal, open it. Enter the chem lab to complete the quest. Feel free to deal with the chemists are you see fit.
Even though you've notched another quest under your belt, how you decided to deal with Nelson and Trish will have non-quest repercussions. You might have to deal with Nelson's dad at the patio of the Colonial Taphouse, or you might have to deal with Marowski, the guy who ran the chem lab you probably destroyed. If you have to deal with the latter, someone will let you know.