Today is the day Ivy Moxam escapes from the cellar, her prison for the last 13 years. After she makes a desperate 999 call from a phone box, Ivy is picked up by the police and taken to be interviewed.

The investigation into a missing ten year-old puts more pressure on Ivy to relive an ordeal she would rather forget. A witness confirms that Mark White is responsible for Phoebe’s kidnap and Carne and Merchant find themselves surrounded by a new team from Scotland Yard.

As the police start to make breakthroughs in their investigation, Ivy turns to her sister Emma, who seizes the chance to connect with her. But they are interrupted by the arrival of a mysterious letter addressed to 'Alison' - White’s name for Ivy.

When evidence of Ivy’s DNA is all that is discovered in the cellar, she is arrested on suspicion of perverting the course of justice and taken into custody.

Ivy is forced to face her past and put herself at risk in order to save Phoebe. Carne and Merchant do everything they can to protect her, but in underestimating Mark White they have left their own lives hanging in the balance.