“A Drink Before the War” opens with detective Patrick Kenzie meeting with three politicians. They are Senator Sterling Mulkern, Senator Brian Paulson, and Representative Jim Vurnan. They tell Patrick that a cleaning woman, Jenna Angeline, has stolen some important documents related to an upcoming street gang bill, and has disappeared along with the documents. They hire Patrick to find Jenna and the documents. Patrick and his partner, Angie Gennaro, begin working on the case. Almost immediately, Patrick notices that someone is watching his apartment. The man is later identified as gang member Curtis Moore.

Patrick and Angie soon find Jenna and they consider turning her over to Mulkern. Patrick, however, feels that there's something the politicians haven't told him. Jenna says she doesn't have documents, but doesn't reveal what she does have. Patrick convinces Angie to wait until they see what Jenna has before calling Mulkern. The next day, Patrick goes with Jenna to the bank where she retrieves a photograph of Senator Paulson, wearing only his underwear, in a motel room with a black man later identified as Marion Socia. Before Jenna can give Patrick any more information, including the location of the rest of the photos, Curtis Moore kills her. Patrick shoots Curtis. He's taken in for questioning but doesn't reveal the photo Jenna gave him.

Patrick and Angie soon learn that Marion Socia is Jenna's husband, either legally or common-law. Socia is the leader of the gang called the Raven Saints. Curtis Moore is one of Socia's trusted lieutenants. At the funeral, Socia stands on one side of the coffin. Roland, son of Socia and Jenna, stands on the other with his rival gang, the Avenging Angels. The two gangs had worked together until recently, but were now headed for an all-out war.

That war erupts with dead on both sides. Curtis Moore is in the hospital when someone kills him and carves Jenna's initials in his skin. Patrick and Angie continue to look for the rest of the photos and they eventually realize that Jenna had a car. They find a locker key in the car and begin trying to fit it into lockers all over the city.

They are at South Street, a busy train station, when they find the right locker. They are both surprised when the key turns and leave the locker corridor with bag clearly in sight. They are attacked in the open lobby but their friend, Bubba Rogowski, comes to their aid until he's hit. The police arrive and Patrick and Angie run with their package. Back at their office, they discover the photos are of Paulson raping a young black boy while Socia helps. Roland and members of his gang chase Patrick and Angie, and that's when Patrick gets a good look at Roland so that he realizes Roland is the rape victim in the photos. He doesn't want the photos made public because he'd lose his credibility with his gang.

Roland's gang gains momentum and Socia loses most of his followers. During a confrontation, Patrick and Angie shoot and kill Socia. They then meet with Mulkern and convince him to pay their bill as they hand over the photos. They keep one picture back, having already given it to a reporter, Richie Colgan. The story soon hits the papers and police call for Paulson to turn himself in. Patrick and Angie meet briefly with Roland. He says he didn't ask for any favors but no longer has any interest in killing Angie and Patrick.