Stranded on a parallel Earth, the Tenth Doctor finds himself mentoring a traumatized child soldier enlisted in a hopeless war to save humanity. With an endless number of mysteries in sight -- from the enigmatic alien invaders they call Angels, to the strange machines known as Evangelions used in the planet's defense, to the shadowy maneuvers of those in power -- the Doctor is drawn deeper and deeper into a web of mistakes and sins that threaten to consume all humankind. In time, he'll face the darkest question of them all: what if there are some worlds -- and some children -- even the Doctor can't save?