Mac, Lindsay, and Flack are called to a crime scene where a security guard is found stabbed to death by a stairway. Investigating the surrounding area, however, Mac discovers a bomb. Before he and Flack can escape the building, after leading the inhabitants to safety, the bomb goes off, severely injuring Flack and several others. Amid the following chaos and involvement of Homeland Security, as well as being contacted by the bomber who threatens to bomb another location, Mac recalls a traumatic event from his Marine days when he was serving in Beirut in 1983.