The first stone henge to be discovered in Britain for a century would be cause enough for major celebration. But there's double bubbles as Tony Robinson and his hardy team of archaeologists celebrate their 200th dig. Jane Marchand from Dartmoor National Park Authority was alerted by a walker to standing stones peering out of an East Devon reservoir at low level. This is Francis Pryor's dream site, but Mick has also been interested in Dartmoor for some time. They have stone circles, stone rows and cairns apparently dating from 3000 to 1500 BC. There is a central mound which interests Francis, and which Phil thinks is Stone Age, thus pre-dating the other monuments. But the cairns may be recent, throwing into doubt the dating of the other features. This is cultivated farmland, atypical of Dartmoor's usual bleak landscapes. John is dubious about getting any meaningful geophysics results, but proposes nevertheless to wheel his trolley through the mud. Stewart and Henry create a 3D image of the prehistoric landscape. Phil teaches Matt his favourite activity, flint-knapping.