A pile of bones has been discovered in a narrow cave. They include cows, dogs and a human skull, which have been dated to the late Iron Age Celts. Local archaeologist Mark Horton is keen to discover whether this is a ritual site or just a rubbish pit; so Carenza joins a team of cavers to find out. Every handful of mud must be hauled out and sorted. Meanwhile bone experts Andy Currant and Margaret Cox examine the finds so far. One female skull shows clear evidence of a violent death; and another elderly female was suffering from Paget's disease. The quantity of dog bones may indicate an ancient dog cult, as described by Richard Massey from English Heritage. Archaeometallurgist Andrew Lacey casts a bronze dog model, designed by Victor. In spite of strong geophysics, the surface digs initially show no archaeology at all. Back in the lab, Margaret and Andy make a macabre discovery. Celtic expert Miranda Aldhouse-Green suspects this all hints at human sacrifice.