Tom Barnaby joins the Causton Chamber of Commerce trip to Brighton as he is suspicious of dodgy mayor Hicks' plan to buy coastal land on which to buy chalets. Also opposed to the plan is Lady Matilda Williamwho has had threatening letters as have the local vicar oily property developer Hugh Dalgleish and hotelier Jenny Russell. Lady Matilda is descended from Richard of Guillaumea ruthless contemporary of William the Conquerorand believes that her son Richardlast of the line and brain-damaged for twenty yearswas the victim of ancestral jealousy.In Brighton Dalgleish is murdered with the sword stolen from Guillaume's tomb and Hicks is arrested for fraud. Having learnt how Richard came by his accident Barnaby is convinced that Richard and not the property scam is the cause of yet another victim being claimed by the sword of Guillaume.