When the redoubtable Sir Horace Stanton-Lacy is ordered to South America on business, he leaves his only daughter Sophia with his sister, Elizabeth Rivenhall, in Berkeley Square. Newly arrived from her tour of the Continent, Sophy invites herself into the circle of her relatives. When Lady Ombersley agrees to take in her young niece, no one expects Sophy, who sweeps in and immediately takes the town by storm. Beautiful, gay, impulsive, shockingly direct, Sophy sweeps into elegant London society and scatters conventions and traditions before her like wisps in a windstorm. Resourceful, adventurous and utterly indefatigable, Sophy is hardly the mild-mannered girl that the Rivenhalls expect when they agree to take her in. Kind-hearted Aunt Lizzy is shocked, and her arrogant stern cousin Charles Rivenhall, the Ombersley heir, vows to rid his family of her meddlesome ways by marrying her off.
But vibrant and irrepressible Sophy was no stranger to managing delicate situations. After all, she'd been keeping opportunistic females away from her widowed father for years. Staying with her relatives could be her biggest challenge yet. But Sophy discovers that her aunt's family is in desperate need of her talent for setting everything right: her aunt's husband is of no use at all, her ruthlessly handsome cousin Charles has tyrannical tendencies that are being aggravated by his pedantic bluestocking fiancee Eugenia Wraxton; her lovely cousin Cecelia is smitten with an utterly unsuitable suitor, a beautiful but feather-brained poet; her cousin Herbert is in dire financial straits and has fallen foul of a money-lender; and the younger children are in desperate need of some fun and freedom, and Sophy's arrived just in time to save them all.
With her inimitable mixture of exuberance and grace, Sophy becomes the mainstay of her hilariously bedeviled family, as a horsewoman, social leader and above all, as an ingenious match-maker. Using her signature unorthodox methods, Sophy sets out to solve all of their problems. By the time she's done, Sophy has commandeered household and Charles's horses, but she finds herself increasingly drawn to her eldest cousin. Could it be that the Grand Sophy had finally met her match? Can she really be falling in love with him, and he with her? And what of his betrothal to grim Eugenia?