Marie Antoine is the charismatic, spoiled daughter of a sugar baron. At age twelve, with her pile of blond curls and unparalleled sense of whimsy, she's the leader of all the children in the Golden Mile, the affluent strip of nineteenth-century Montreal where powerful families live. Until one day in 1873, when Sadie Arnett, dark-haired, sly and brilliant, moves to the neighborhood.

Marie and Sadie are immediately inseparable. United by their passion and intensity, they attract and repel each other in ways that set them both on fire. Marie, with her bubbly charm, sees all the pleasure of the world, whereas Sadie's obsession with darkness is all-consuming. Soon, their childlike games take on the thrill of danger and then become deadly.

Forced to separate, the girls spend their teenage years engaging in acts of alternating innocence and depravity, until a singular event unites them once more, with devastating effects.