'I'm taking a revolver to Thrackley. You never know with blokes like Carson. I hate these harmless, potty people - they're always up to something.' Jim Henderson is one of six guests summoned by the mysterious Edwin Carson, a collector of precious stones, to a weekend party at his country house, Thrackley. The house is gloomy and forbidding but the party is warm and hospitable - except for the presence of Jacobson, the sinister butler. The other guests are wealthy people draped in jewels; Jim cannot imagine why he belongs in such company. After a weekend of adventure - with attempted robbery and a vanishing guest - secrets come to light and Jim unravels a mystery from his past.