巨怪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 William,who 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 Guillaume,a ruthless contemporary of William the Conqueror,and believes that her son Richard,last of the line and brain-damaged for twenty years,was 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.