THIS LATEST intrigue by Geoffrey Household begins as many do -- with an inadvertent stumble into sinister circumstances. One Piers Colet, a mild-mannered historian specializing in ancient economies, is exploring a marshy estuary of the Severn River in Britain and needs lodging for the night. The only proximate room, it transpires, is at Broom Lodge, a remote commune of farmers and metalworkers preparing for Armageddon. They toil and believe under the spell of an engaging but somehow malevolent shaman named Simeon Marrin.

Colet lingers, more curious than wary, and begins to wonder about the source of Broom Lodge's remarkable income; farming it isn't. His wondering hardens into suspicion as he observes Marrin's surreptitious excursions to the estuary by night, and his suspicion hardens into terror when Marrin nearly succeeds in doing him in. Said estuary happens to be the subject of Colet's scholarly researches, so he is able to put two and two together: could there be treasure in some long-abandoned Roman harbor on the Severn? Could there, indeed.

With a knack worthy of James Bond himself, Colet finds eager confederates in the lovely Elsa Marrin, Simeon's disaffected niece, and in the dizzy Major Denzil Matravors-Drummond (Ret.), a Broom Lodge disciple only of convenience. Together they connive to expose Simeon Marrin's secret and the black magic of his followers, and a bit of priceless submarine antiquity besides.