Celebrating her 45th birthday at her local pub, podcaster Alix Summers crosses paths with an unassuming woman called Josie Fair.

Josie is also celebrating her 45th birthday. They are, in fact birthday twins. A few days later, Alix and Josie bump into each other again, this time outside Alix's children's school. Josie has been listening to Alix's podcasts and thinks she might be an interesting subject for Alix's series.

She is, she tells Alix, on the cusp of great changes in her life. Alix agrees to a trial interview. Josie's life appears to be strange and complicated, and although Alix finds her unsettling, she can't quite resist the temptation to keep digging. Slowly Alix starts to realise that Josie has been hiding some very dark secrets, and before she knows it Josie has inveigled her way into Alix's life - and into her home.

Soon she begins to wonder who is Josie Fair? And what has she done?