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.

After four decades of searching for him, they got a one hundred percent match to the Golden State Killer's DNA. As 72-year-old former police officer Joe DeAngelo's arrest unfolds in real time, chilling facts materialize that illuminate Michelle McNamara's prescience in her book's epilogue.

As their high-profile trial begins, Willy and Sal attempt to neutralize witnesses against them in a variety of ways — some legal, others not so much.

In the wake of a stunning verdict, questions arise about the jury's credibility, and one juror in particular soon adopts a very creative legal defense.

Sal takes uncharacteristically desperate measures in the face of new federal charges, an unearthed paper trail and increasing pressure on a confidant.

Former students of Archbishop Keough High School reveal allegations of sexual abuse by Father Joseph Maskell, the chaplain of the school. It details the personal story of 'Jane Doe' (Jean Hargadon Wehner) who claims that Father Maskell took her to see the dead body of Sister Catherine Cesnik in Lansdowne, Maryland.