On a rainy June morning, tens of thousands of people crowd into Duluth for the city's biggest annual event: Grandma's Marathon. Exhausted runners push to reach the finish line and spectators line the streets to cheer them on. Then, in a terrifying echo of the Boston Marathon bombing, there is an explosion along the race course, leaving many people dead and injured. Within minutes, an urgent manhunt begins for the person who planted the backpack filled with explosives.

Stride, Serena, and Maggie are forced into supporting roles in this investigation as FBI special agents take over the case. Politicians and the media push for answers and rumors fly as people leap to the conclusion that this was radical Islamic terrorism.

One spectator remembers being jostled by a young man with a backpack not far from the bomb site. He spots a young Muslim man in a tourist's photo of the event, convinced that this man bumped into him in the crowd-but there's no backpack in the photo. Distrusting that the police will follow up, he tweets the photograph to the public. In seconds, the young man, Mehdi, becomes the most wanted man in the city.

When new photographs surface from the day of the bombing, the spectator realizes he may have made a terrible mistake. Stride, Serena, and Maggie awaken to the possibility that the motive behind this bombing may not have been terrorism at all. But can they find the truth before more innocent people are killed?