Detective Sean O'Reilly is called in to interrogate a young boy who is the only witness to a massacre during a gang war. The child's story is too far-fetched for most of the Chicago Police Force to believe - claims of an army of other-worldly creatures who attacked and dismembered the rival gangs. But Sean O'Reilly believes him, because he's seen them himself. An encounter he had as a boy in a dark forest in Ireland did not just given him nightmares, but changed his entire life.