Rosie Yates is now 13 and is starting to display increasingly challenging, emotional and physical behaviour. Veering wildly from joyous obsessions with her letter X, tigers with Xs on their nose, ticket barriers and Fedex lorries to sudden violent outbursts both directed at her parents Si and Emily and herself.

The family realise that Rosie is simply struggling to cope with the onset of puberty, just like any other teenage girl. Whilst the family deal with a teenage Rosie, their geneticist calls with news of Rosie's condition.

In the earlier timeline Emily starts thinking about whether she and Si should think about having another child after a chance meeting with a mother of a child with a chromosomal disorder.