After sustaining broken ribs on the job, Mac is at home recovering but finds himself unexpectedly working from home when a murder mystery occurs right in his own neighborhood. The exchange of a briefcase with a mysterious canister of liquid in the apartment across the courtyard seems odd enough, but when the messenger of the case is found dead, Mac, knows that something isn't right with his neighbor. Mac is equally surprised when he sees his old girlfriend, Peyton Driscoll, staying in the neighbor's apartment, a man he suspects of killing the victim. Peyton initially defends her friend, who she considers a respected Professor of Mathematics, but when Mac proves that he has lied to her and that traces of Sarin are found in a dead bird that the suspect used to have in his apartment, Peyton quickly changes her mind. The Professor, who is a well-known conspiracy theorist, set out to show how easily terrorists could launch a chemical attack on U.S soil by doing it himself. The victim supplied the Professor with the Sarin and in the process managed to poison himself. Mac finds evidence in the suspect’s apartment that leads them to the site the Professor plans to release the poison, the Manhattan Conservatory, at an event he was to attend with Peyton. Mac and the team rush to the Conservatory to stop the Professor and save Peyton and hundreds of others from a horrific death.
Mac was surprised to see Peyton again and working this case together has both taking a renewed interest in one another.