In this film, military historian Mike Loades investigates one of the most impressive and successful creations of the Napoleonic Wars: The Lines of Torres Vedras. Mike explores the spectacular lines of forts that ran from coast to coast across the neck of the Lisbon Peninsular, defending the Portuguese capital from a land attack by the French. They were constructed under the orders of Sir Arthur Wellesley, Viscount (later Duke of) Wellington, in anticipation of a third invasion of Portugal by Napoleon's forces. The British army under Wellington was there to stand with its old ally.

Working with historians and Portuguese re-enactors, Mike discovers how the Lines were designed and built - and also the ingenious communications system that was set up to enable the forts and troops to stay in touch under attack. He gets on horseback to find out how Wellington commanded his troops and the building of the Lines whilst preparations were made to withstand the invasion

In a vast enterprise involving thousands of Portuguese civilians, 126 forts were built between October 1809 and October 1810. They made use of the natural landscape, fortifying the summits of two chains of hills. From these strongholds they could defend the roads that ran through the passes with artillery.

Mike also discovers how defeating the French entailed great suffering by the Portuguese people - Wellington ordered a scorched earth policy north of the Lines and the forced evacuation of the entire civilian population. The aim was to make it impossible for a French army to live off the land and to starve them out of Portugal.

When a large French army, under the command of Marshal Massena, arrived at the Lines in October 1810, Wellington's plan was put to the test...