Disillusioned working-class teenager Mona forms a close friendship with Tamsin, a wealthy girl neglected by her parents and troubled by her sister's death. Their bond develops into an intense, secret affair, provoking a violent reaction in Mona's devoutly religious ex-convict brother. Drama based on Helen Cross's novel, starring Nathalie Press, Emily Blunt and Paddy Considine.

In the Yorkshire countryside, working-class tomboy Mona meets the exotic, pampered Tamsin. To seal their friendship, Mona introduces Tamsin to her born-again Christian brother and helps her spy on her adulterous father. Bound together by their secrets, the two girls see their friendship deepen and enter into dangerous waters.