Charlotte Ashford will do anything for her cousin Lydia. They are practically sisters, growing up together in the heart of Victorian England. They dreamed they would share their lives forever and made girlish promises to always be there for each other. When Charlotte's path veers to America and Lydia marries the enchanting Nathanial Hammond, their dream of sharing a life together is fractured.

Years later, Charlotte is unexpectedly left penniless and alone. Learning of her cousin's fate, Lydia doesn't hesitate to invite Charlotte to return to England and live with her. But when Charlotte arrives, she discovers that not all is as idyllic as it seems for her cousin.

With promises from the past, to always be there for one another, Lydia is pushed to ask Charlotte to embrace a role that she never could have imagined-to become her husband's mistress.

As secrets surface and truths unravel, Charlotte finds herself ensnared in a web of duty and deception. Amidst the intricate dance of desire and loyalty, the picturesque life Charlotte and Lydia once envisioned together, now holds a far more complicated plot-one that beckons them to defy convention, question the boundaries of love, and navigate the perilous territory of a shared life with one man.