Holmes and Watson are staying with Reginald Musgrave, the head of an ancient Sussex family, when his butler, Brunton, suddenly disappears. Brunton (something of a ladykiller and a highly educated man) had been taking an interest in an ancient family tradition called 'the Musgrave Ritual', which centres on a Carolingian riddle. And then a second member of the household vanishes, equally suddenly.
The old riddle proves to be the key to tracing the butler's body and finding the treasure the dead man was after - the crown jewels lost in the English Civil War. And as so often, it's a case of 'cherchez la femme'.