Testing of fingerprints settles the case. One is the victim's, and another is the promoter's. He was handed the bottle before the victim died. Who handed over the bottle? McQuinn, but Det. Angell has to stop the promoter from taking revenge, since he was the intended victim. In the meantime, Mac is waiting for McQuinn's private plane to make an arrest. Chemical burns on McQuinn's hands seal the deal. Mac tells Hawkes the news, and Hawkes wonders if he could have changed things by taking the victim's phone number rather than simply giving her his.
When Peyton and Mac finally meet by the vending machines, she is surprised he shows up after their last conversation, but she knows Mac was protecting the department by doing his job.
This episode was more about the detectives than murder. Although I understand Mac's tirade, did he have to teach by humiliation? Lindsay might have missed the phone number, but she does not ignore evidence which points to a colleague. Case in point, when she learned DNA on a cigarette butt belonged to someone in the lab, she turned test results over to Danny, who went straight to Mac with the news it was his ("Run Silent Run Deep"). Yes, she can overlook the chance someone is guilty without solid proof ("Open and Shut"), but she is learning that a person's character is part of the deal. Of all people, Hawkes should know better than to leave a case open for a defense attorney to blow apart. He has testified often enough. Still, he also knows the same attorney can fall down on the job ("Hung Out To Dry").