The reclusive lives of elderly eccentrics William and Mary Bingham comes under police scrutiny when a social services investigator is murdered. DCI Barnaby and DS Jones must unearth generations of family secrets and decipher astronomical charts to find the killer. Meanwhile, Barnaby's wife Sarah arrives in Midsomer.

DS Jones goes undercover at the Oblong Foundation after one of the cult's young female members disappears suddenly. But he and DCI Barnaby soon start digging into an old case involving the death of a couple in a boat explosion.