Timothy West and Prunella Scales return to Wales to explore a canal that has eluded them for over half a century. The famously beautiful Monmouthshire and Brecon canal is cut off from the rest of Britain's network of waterways by the Black Mountains. The isolated gem meanders its way through the tranquil Usk valley. Tim and Pru visit the old iron works at Blaenavon, where children as young as five were once put to work; and the beautiful 15th-century castle at Tretower - rumoured to be haunted. They also lend a hand with some lock restoration work.