On a rainy September night in 1920, Brenna Caffrey, her husband and their two young sons arrive at Liverpool docks from Ireland. They have come in search of a better life, but there is no one to meet them, as had been arranged, and they have nowhere to live. Then Brenna, who is expecting her third child, goes into labour. Collapsing on the steps of a grand townhouse, she is taken in by kindly Nancy Gates, housemaid to the wealthy Allardyce family who live there. 

          Upstairs, the unhappily married Eleanor Allardyce is also struggling to give birth. Both women produce daughters that night – Cara and Sybil, the September Girls. Enemies at first, Brenna and Eleanor eventually become friends, but friendship between their daughters is another matter. 

          Nineteen years later, at the start of the Second World War, Cara and Sybil find themselves thrown together in Malta, military key to the Mediterranean. It is a time that has life-changing repercussions for them both, while back home in Liverpool, the bombs pour down on a defiant city.