The CSIs face their toughest challenge yet when a bomber uses random cell phone pages to detonate C-4 bombs across the city, and a member of the team fights for life when seriously injured during one of the explosions. In a race against the clock, the team works with local agencies and government authorities to locate the bomber and narrow down the next trigger among the city's six million cell phones before he strikes again. Meanwhile, as Mac investigates a routine crime scene, he stumbles upon a bag containing C-4 explosives and is unable to evacuate the building quickly enough to get his team out of harms' way. While a CSI is rushed to surgery, Mac struggles with the memories of the last person he was unable to save, a fellow Marine killed during the 1983 Beirut barracks bombing.