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?

The Panacea is on a two hundred year journey to carry the last of humanity to their new planet. Two groups were chosen to be saved- the engineers who built this ship, and the Evves, who were chosen for their genetic uniqueness. There always has been conflicts between them, but when Sapho, one of the engineers, finds out that their destination planet can't support everyone, she realizes some people must be sacrificed.