Mac chases the 333 stalker to Chicago and evidence leads him to the Chicago Tribune Building, where he finds a thirty-year-old corpse. Mac learns the victim is the man who killed his closest childhood friend: a murder Mac witnessed when he was a teenager in room 333 of a hotel, and could have prevented.
Back in NYC, while Stella and the team investigate a subway conductor's body found on the train, the train speeds off on a wild ride with the team inside. In a close call, they bring the train to a stop. As they piece together evidence, they discover Stella's so-called admirer Drew is the 333 stalker and the brother of Mac's murdered childhood friend. Upon Mac's return to NYC, he is ambushed by Drew and taken to an abandoned City Hall train station. Realizing Drew blames Mac for his brother's death, the CSIs race to find him before it is too late.
