The series follows the tragedy that occurred when terrorists attacked at the annual Boston Marathon on April 15, 2013. They carried out the attack by placing two homemade pressure cooker bombs which detonated near the finish line of the race 14 seconds and 210 yards (190 m) apart. That resulted in three fatalities and hundreds injured, including 17 who lost limbs. With the help of archive footage, interviews with investigators, witnesses, survivors, and occasional reenactments, it offers details about the FBI investigation into what was quickly deemed a domestic terrorist attack.
In the first episode, hours after the blasts, investigators pore over evidence. One photo and a few seconds of footage lead to a breakthrough - and an agonizing choice.

The images of brothers Tamerlan and Dzhokhar Tsarnaev are released to the public setting off a violent chain of events. Meanwhile, clues from their past tell a complicated story - how was the radicalization of the ethnically Chechen brothers and the existence of potential collaborators. The post-9/11 biases played a role in the investigation.

Following their identification, the Tsarnaev brothers killed an MIT policeman, then kidnapped a man in his car. They had a shootout with the police in nearby Watertown, during which two officers were severely injured (one of whom, Dennis Simmonds, died a year later). Tamerlan was shot several times, and his brother Dzhokhar ran him over while escaping in the stolen car. Tamerlan died soon after.
An unprecedented manhunt for Dzhokhar ensued, with thousands of law enforcement officers searching a 20-block area of Watertown. Residents of Watertown and surrounding communities were asked to stay indoors, and the transportation system and most businesses and public places closed. After a Watertown resident discovered him hiding in a boat in his backyard, Dzhokhar Tsarnaev is cornered into an intense standoff. As officials navigate remaining questions and suspicions, the city emerges Boston Strong.