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 realises 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 . . .