The day the First World War broke out, Alfie Summerfield's father promised he wouldn't go away to fight, but he broke that promise the following day. Four years later, Alfie doesn't know where his father might be, other than that he's away on a special, secret mission. Then, while shining shoes at King's Cross Station, Alfie unexpectedly sees his father's name on a sheaf of papers belonging to a military doctor. Bewildered and confused, Alfie realizes his father is in a hospital close by—a hospital treating soldiers with an unusual condition. Alfie is determined to rescue his father from this strange, unnerving place.