Sweet Danger is a crime novel by Margery Allingham, first published in October 1933, in the United Kingdom by Heinemann, London and in the United States by The Crime Club as Kingdom of Death; later US versions used the title The Fear Sign. It is the fifth adventure of the mysterious Albert Campion, aided as usual by his butler/valet/bodyguard Magersfontein Lugg, and introduces the recurring character of Amanda Fitton.

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. Following their adversaries, headed by a secretive financier named Savanake, they travel to the village of Pontisbright in Suffolk, former seat of the heirs to the crown of Averna.

They spend a night in the local inn, the Gauntlett, during which Lugg sees a corpse wrapped in a winding cloth on the nearby heath. 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. 17-year-old Amanda, the middle child, has an interest in electricity and radio, and runs a generator from the watermill. Aunt Hattie recently disturbed a burglar in the house, who Campion immediately identifies as "Peaky" Doyle, an old associate of Savanake who recently shot at Campion on the continent.

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. Back at the house, they find a drunken Lugg and Scatty Williams, the Fittons' servant, have caught Peaky Doyle breaking into the mill and have beaten him up. He had been found looking at the inscription. Campion reveals that he informed Doyle of the inscription in hopes of getting some help solving the riddle, and insists they leave Doyle unconscious on the heath,

Campion is called to a meeting with Savanake, who insists he take a job in Peru, leaving immediately; when Campion's friends receive a note saying he has run out on them, they resolve to stay to finish the business, but are at a loss as to how to continue. The mill-house is invaded, and everyone left bound and gagged in the dark while it is searched, but they are mysteriously released later.

A letter arrives addressed to Campion, which the men read after Amanda has opened it; it describes a drum belonging to the Pontisbrights, currently in a Norwich museum. Amanda reveals she has come into £300, and plans to buy a car and some radio equipment. When Farquharson and Randall arrive at the museum to retrieve the drum, they find Amanda and Scatty Williams have already taken it.

At the mill, Hal, the youngest Fitton, meets Dr Galley, who tells him he has found evidence that Hal is the Pontisbright heir, and insists they all visit him the following night for dinner. Amanda arrives, and Hal locks her in the grain store, taking the drum. When the others return, he goes to tell them he has it, but when they return to his room the skin is gone from the drum and Amanda is free.