Professor Alice Roberts reveals the forgotten story of the Roman Army's secret weapon in Britain - their cavalry. These fearsome horsemen were the key to defending Britain's most famous Roman monument fortification. Alice sets off across Hadrian's Wall to investigate any evidence the Roman cavalry left behind, while a team of archaeologists and historical re-enactors attempt to re-stage a Roman cavalry tournament - a spectacle that no one has seen for over 1,600 years. Alice joins them at a public display in Carlisle where 30 riders perform in front of a crowd of spectators. The film also explores the latest archaeological digs happening across the UK, each of which is searching for new evidence of the Roman cavalry. Alice visits some of the most iconic sites associated with the Roman cavalry, including Chester's Roman fort, Vindolanda fort and museum and Hexham Abbey. Along the way she builds a picture of the horsemen's lives here on the northern frontier of the Roman Empire.