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. Jonathan 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.