1904, London. Lydia Daye adores her husband John, but his secret government job means he is often away. She consoles herself with her small son, Adam, and the knowledge that John's salary allows them to live comfortably. Dolly Ellerway lives just two miles away, but in a different world. Pregnant, she is delighted when the father of her child says he'll marry her, even though he can barely afford it. It seems unlikely that the two will ever meet, but one day Lydia sends a fateful letter that will change both women's lives forever….