Fin and Thomo return to England with the seemingly simple purpose of delivering Sir Robert de Lacy, lately wounded at the battle of Vitoria, and his wife to their Dorset estate, but this is only a deception arranged by the Duke of Wellington commanding the army in Spain for his desire to know whether the new experimental weapon, Congreve's rockets, has improved sufficiently for deployment with his army that is much in need of artillery.
Fin and Thomo's investigation will take them to London, where they have to prove their worth to members of the Rocket Brigade, survive the ravages of storms at sea before setting foot on dry land to begin the campaign, under a foreign commander who once served Napoleon, leading to the climactic Battle of Leipzig, where not all of the new troops will live to fight another day...