Bullard asks Barnaby to investigate the disappearance of a friend's daughter who was a member of the Oblong Foundation, a cult based at Malham Hall. Jones goes undercover as a new recruit to solve the mystery, but he and Barnaby soon start digging into an old case involving the death of a couple in a boat explosion, which could be linked to the missing woman.

With Will Knightley, Kevin Doyle, Holly Aird, Barbara Flynn and John Woodvine