In Boston, a vibrant city prepares for the annual Boston Marathon, one of the most significant running events in the country. In the heart of this celebration, brothers Dzhokhar and Tamerlan Tsarnaev set off homemade bombs near the finish line, causing widespread chaos and fear. The explosions kill three spectators and injure hundreds of others, turning a day of athletic triumph into one of tragedy. Amid the panic, Boston Police Sergeant Tommy Saunders, a composite character representing the law enforcement officers who worked that day, finds himself at the epicenter of the emergency response.

Law enforcement agencies, led by FBI Special Agent Richard DesLauriers and Boston Police Commissioner Ed Davis, quickly mobilize a massive manhunt to capture the perpetrators. Investigators work tirelessly, sifting through photographic evidence and piecing together the clues amidst the shock and turmoil that grips the city. Cooperation between various departments and the utilization of state-of-the-art forensic technology are key components as they race against time to prevent further terror while the city is on edge. Meanwhile, the entire Boston community comes together in a show of resilience and unity, embodying the spirit captured by the rallying cry "Boston Strong".