Mac tells Lindsay about a cold case from twenty years ago in which 2nd grader Tommy Lewis went missing while walking his dog to the neighborhood park. While everyone suspected teenage troublemaker Keith Milner of killing Tommy when he was found in the park holding Tommy's bloody jacket and dog, the police did not have enough evidence to charge him. In the present, Mac and Lindsay arrive at the same park to find Milner murdered. Lindsay lifts black smudges off of Milner's jacket and reconstructs them to form a fingerprint that matches Nathan Lewis, Tommy's father. When the team learns that Nathan is missing, Det. Lovato speaks with Tommy's mother April who plays a voicemail from Nathan from the night before that makes it clear he was intoxicated and angry about seeing Milner at the local bar. They find Nathan but he is unable to remember anything beyond following Milner into the alley by the bar and roughing him up. They speak with Keith's wife and she reveals that her husband documented his resolve to clear his name for the crime in his journal. Dr. Hawkes connects four substances found on Milner's neck wound as constituents in a specific cheese called Flaming Nettle Gouda. They determine the closest store to sell this type is a neighborhood deli owned by Mitch Ventri, a man who had often helped the Lewis family. When they question Mitch, he confesses to assaulting and killing Tommy and directs them to the boy's body in his basement. Mitch explains that awful night 20 years ago when a drugged up Milner witnessed Tommy going into the deli and told his parents but no one believed him. Mitch killed Milner when the latter confronted him asking him to come clean. Flack and his sister disperse their father's ashes at Citi Field after they find a letter from him with the request. Jo questions Mac about his condition when she picks up on signs of his aphasia.