Having survived the Battle of Leipzig, the newly created Rocket Brigade has been split with Fin and Thomo briefly returning to England before heading back to the Duke of Wellington where they are to report on this new weapon. The new troop have to pass a series of tests and conditions before being allowed to take part in the invasion of France. Assisting partisans; discovering a wrecked ship: being isolated on the wrong side of a river with the garrison of Bayonne approaching: providing the only ordnance halfway up the hillside overlooking Toulouse, Fin and Thomo have to find a way out of the blunders created by those in command, leading them, as always, into plenty of adventures where they meet old and make new friends along the way...