Following his first, supporting appearance in The Crime at Black Dudley (1929), it is the first of many novels starring the mysterious Albert Campion, and introduces his butler/valet/bodyguard Magersfontein Lugg.

Apparent buffoon Albert Campion offers the family sanctuary with his friends in remote Suffolk, but a local commits suicide, the Judge vanishes, and another disappearance follows soon after.

The night they arrive, a roving fortune-teller visits, and soon afterward the local Rector "St." Swithin Cush, a mild and much-loved man, commits gruesome suicide, leaving a note and some mysterious clues—a red knight from his chess set, and the word "Danger" in encrypted form.

They search for the judge's secret knowledge, the clue to the identity of crime boss Simister, which has brought such danger, but find only a large box of children's books.

They go there, fetch the judge, and he, Giles and Campion decamp for Mystery Mile once more, where they find the household, including Mr Barber, drugged.

Recuperating later, Campion's friend Stanislaus Oates tells him the secret of Swithin Cush's suicide – he was in fact not a parson at all, having taken the place of his brother who died young.

Biddy and Giles Paget plan to marry Marlowe and Isobel Lobbett, respectively, and to sell the valuable painting for a small fortune.