Hattie and Martyn are a decent, hard-working, ecologically-minded young couple – partners with a new baby, bickering over whether they should get married or not and how to arrange their lives in a morally sustainable way. Hattie has given up work to look after baby Kitty, but she is, frankly, bored by domesticity. They meet Agnieszka, a Polish domestic paragon who's married to a bus driver back home and is sending money back to her old mother and sister. Morally responsible couple that they are, Martyn and Hattie take pity and invite her to live with them.

Hattie eventually succumbs to temptation and asks Agnieszka to baby-sit, and soon she’s back at full-time work. In fact life is pretty much as it was before the baby came along, except the house is cleaner and better organized, and she's galloping ahead in her career. Martyn is thrilled and wants a marriage ceremony but Hattie refuses: life is perfect as it is, thanks to the existence of Agnieszka, who is modest, docile, prepared to work for a pittance, and not even too pretty for comfort. And if she tells the occasional lie – the little sister turns out to be a child – and her social attitudes are atrocious, well, they can be overlooked. She'll learn our ways along with the English language.

But soon, things begin to sour. Martyn and Agnieszka grow closer and it occurs to Hattie now that baby Kitty, given a choice, would choose Agnieszka over her. Her friends are warning her – but that's too vulgar for consideration. And their lives are by now hopelessly intertwined – it's too late. And so the downward spiral continues as visas, a marriage of convenience and even her own friends and family conspire to force Hattie out of her home and her partner's bed …