When Averna, a tiny but oil-rich principality on the Adriatic Sea, becomes a vital port after an earthquake, Albert Campion is called in to track down proof that the land belongs to an aristocratic family, believed long died out.

With an unscrupulous financier and his hired thugs also on the trail, Campion and his confederates must unravel the mystery, while defending the Fitton family, claimants to the title, in the strange Suffolk village of Pontisbright.

Guffy Randall is surprised to find his old friend Albert Campion in a French hotel, accompanied by Eager-Wright, a mountaineer, and Farquharson, a prospector, masquerading as minor royalty.

When Campion explains that they are seeking proof of British rights to a small territory, recently disturbed by an earthquake and turned into a strategically important harbour with its own fuel, Randall joins the team.

They move to the local mill, which is run by impoverished Fitton family, who claim to be the lost heirs to the Earldom of Pontisbright: siblings Mary, Amanda, and Hal, and Aunt Hattie.

Amanda shows them an ancient inscription on a slab of oak cut from the Pontisbright estate, which speaks of a crown hidden by a split diamond, and other proofs, believed to be the missing deeds and titles, marked by a bell and a drum.

Campion and his friends visit Dr Galley, the local medic, a bizarre, eccentric old man who tries to scare them back to London with talk of a curse on the village.

Aunt Hattie disturbs a man rifling her jewellery, which leads to a gunfight in the yard; the thief is driven back into the house by Campion, dressed as a woman.

He tells them he arranged for the two men to be arrested by friends, so they could safely take the drum-skin to London: it bears information about deeds giving title to Averna.

He unveils Doyle, bizarrely dressed and near death, just as the sound of an enormous bell rings loud around the valley; the others overpower Galley, lock him up and flee in a camouflaged boat prepared by Amanda.