Iolanthe has an extraordinary family: eccentric, chaotic and bursting with life and warmth. Waifs and strays (children or animal) are always appearing at the door, and while her father charms them with rambunctious childishness, it is her kind and gentle mother who they inevitably fall in love with and never want to leave.
One evening, Agata, their Polish neighbour, arrives uninvited on the doorstep. With her food and wisdom and cleaning abilities she becomes a permanent fixture in the home, dispensing advice and looking after their increasingly fragile mother. And then Carol arrives, Iolanthe's age and more in need of the family than any previous stray and the two girls become inseparable, growing up as sisters. But both Agata and Carol have murky secrets in their past, and Agata refuses to trust that the changeling is above lying and treachery when it suits her.
As Iolanthe and Carol grow up, Iolanthe begins to wonder how well she ever knew her foster sister, and soon her loyalties are tested to destruction in order to save her parents' marriage, and the family itself.