It is a very strange thing indeed. Lay out a map of Britain and something odd becomes apparent: at 1 degree 50 minutes west there is a dead straight line going true north from Stonehenge to Lindisfarne. Two of Britain's most-ancient and most-revered sites. But there is more . . .

Starting in the south, the line goes dead north from Castle Ditches to old Sarum, through Stonehenge, Knap Hill and Avebury stone circles, past ancient places in Cricklade, Bibury, Notgrove and Wootton Wawen. It goes through Long Low, Thor's Cave, Mam Tor, nearby Twelve Apostles stone circle, Ilkley Roman site, The Badger stone, ancient sites in Lanchester, Eochester, Millstone Burn, Bewick Hill and all the way to finish plum on Lindisfarne Island - one of the most ancient of early Christian sites and still a retreat and monastery.

But the significance of the Great North Line has been forgotten over time, and no one has ever walked the whole route before - at least not in the last 1,000 years. Robert Twigger has travelled this ancient line on foot, like a biologist walking a transect. Sticking as closely as possible to the meridian, he answers the questions raised by this most mysterious and ancient of British lines and discovers if it still has a power over us today.