For the past several years Sophia Stanton-Lacy (known as Sophy to everyone) has lived away from England, following her diplomat father Sir Horace around Europe while the Napoleonic Wars raged on. Now that the Battle of Waterloo is over and Napoleon has once again been exiled, her father receives a temporary post in South America. Instead of taking his daughter along, he asks his sister Lady Ombersley to watch over his "little Sophy" and help find her a husband. However, "little Sophy" is nothing like anyone expected. 5'9" in her stockings, she is outgoing, chic, and quite independent, taking the town by storm with her unconventional manner.
Though most of her cousins take to her on sight, her autocratic cousin Charles Rivenhall, forced by his father's debt to shoulder the family finances, resents the disruption by his lively and confident cousin of what has become, in all but name, his household. With Charles encouraged in domestic tyranny by his spiteful fiancée, Miss Eugenia Wraxton, Sophy and Charles begin a battle of wills. Soon after her arrival, Sophy realizes that all is not well in the Rivenhall household and proceeds to solve the various problems of the family with her trademark flair, saving her cousin Hubert from a moneylender, arranging through an involved and hilarious scheme her cousin Cecilia's extraction from her infatuation with (and later engagement to) a poet, and promoting her marriage to the eligible Lord Charlbury, the man favored by her brother and parents and ultimately, the man that Cecelia discovers she loves.
Slowly, much to the consternation of them both, Sophy and Charles find themselves falling in love, with Sophy's devilry lightening his dictatorial tendencies. In the end, at the successful conclusion of her incredibly audacious scheme to unite Cecilia and Charlbury and free Rivenhall from his obligations to his fiancée, Rivenhall proposes, with Sophy accepting.