A bomber strikes a Boston commuter train, leaving multiple people dead and many more injured. When the team is called in to help federal agents with the wreckage, they make a gruesome discovery-most of the casualties were children on a field trip. After Jordan takes the first wave of bodies back to the morgue, she begins the horrifying process of helping parents to identify their loved ones.

Meanwhile, Woody, Lu and Nigel search for evidence that may bring them one step closer to the bomber. Tensions run high in the morgue and in the field when Macy and Lu are reminded of painful losses from their past. Yet, there is no time to grieve, another bomb lies somewhere in the city ready to explode, and it is up to Woody and the federal agents to find it.