Based on the clues that Mac's stalker provides him, he goes to his hometown, Chicago and there he gets more clues which help him realize that the whole thing stems from a moment in his youth. Based on DNA evidence, Mac suspects it's his friend who's behind it but he denies it. Meanwhile back in New York the team is placed in danger by the stalker. Because of the DNA evidence Mac says the only possible suspect is his friend's brother but he doesn't know what happened. Mac later realizes his friend's brother and Stella's suitor Drew Bedford are the same. So he goes looking for him but is captured. So the team tries to find him.
Did You Know?
As evidenced with a scene filmed on a bridge that spans the Chicago River, filming also took place in Chicago, Illinois. See more
When Mac draws his Glock while opening the door, you can tell by the trigger position that the gun is not cocked. Police officers normally carry their handguns cocked, and in any event, it is unlikely that a cop going into a potentially dangerous situation would neglect to cock his gun first. See more