A high society amateur detective uses her wits and invisibility as an 'old maid' to protect other women. Lady Augusta Colebrook, 'Gus', is determinedly unmarried, bored by society life and tired of being dismissed at the age of 42.

She and her twin sister, Julia, who is grieving her dead betrothed, need a distraction. One soon presents itself - to rescue their friend's goddaughter, Caroline, from her violent husband. The sisters set out to Caroline's country estate with a plan, but their carriage is accosted by a highwayman. In the scuffle, Gus accidentally shoots and injures the ruffian, only to discover he is Lord Evan Belford, an acquaintance from their past who was charged with murder and exiled to Australia 20 years ago.

What follows is a high adventure full of danger, clever improvisation, heart-racing near misses and a little help from a revived and rather charming Lord Evan.