The narrative centers on a brilliant and elusive female thief operating under the alias Four Square Jane, who targets the corrupt and ultra-wealthy elite of England, notably the unscrupulous Lord Claythorpe and his associates. Leaving her signature four-square mark behind, she executes impossible jewel thefts and elaborate ruses, often funneling her gains to struggling hospitals while systematically exposing financial wrongdoing.
At a lavish house party hosted by the financier Joe Lewinstein, the elusive thief infiltrates the estate disguised as a visiting American detective, slipping past security to empty the rooms of valuable gems. Later, she restores a stolen Venetian armlet directly to a cash-strapped hospital, forcing Claythorpe into a bitter public and private battle of wits.
As the net tightens, Scotland Yard's sharpest investigator, Peter Dawes, attempts to unravel the identity behind the crimes. He traces a web of deception involving forged checks, stolen heirlooms, and a complex plot by Claythorpe to misappropriate the fortune of his ward, Joyce Wilberforce.
Tragedy strikes when Claythorpe's confidential clerk, Remington, is murdered in the office safe room after discovering financial ruin and missing trust funds. Claythorpe attempts to frame Jamieson Steele, an engineer and secret husband to Joyce, but the investigation uncovers deeper corruption within Claythorpe's financial dealings.
The audacious crimes escalate with the high-stakes robbery of the King's Indian mail and a daring disappearance during a society dinner party where an emerald necklace vanishes in total darkness. The police race to piece together the clues left in the wake of the thefts.
The truth behind Four Square Jane is finally revealed when Joyce and Jamieson reclaim her rightful inheritance from Claythorpe's guarded desk. Lord Claythorpe meets a grim end in his cell, having poisoned himself after his embezzlement and murder are fully exposed.
The story concludes with the mastermind safely escaping to South America alongside her husband, leaving Peter Dawes with a respectful parting letter from Four Square Jane herself, closing the case on one of England's most charming and baffling criminals.
