The year is 1941 and England is preparing for a German invasion. Septimus, an exasperating seventh child, is greatly disturbed by the news that one of his older brothers has just been killed in the war. He 'liberates' a friend's bike at school and runs away, pedalling aimlessly south toward the English Channel, blythely unaware that he is heading straight into a heavily fortified war zone.

After a series of curious encounters, he is run down by a strange man driving a Buggati, a man who has long been believed dead. The man takes him home to mend his banged-up leg. But 'home' turns out to be a secret headquarters. When Septimus has recovered, The man takes him on as a message runner. The rebel in Septimus is gradually tamed by his growing awe of the man, which leads to adventures both dangerous and ultimately deadly.