When upper-class Tamsin (Emily Blunt) meets working-class Mona (Natalie Press) they are immediately drawn to each other. Although coming from different worlds, the two find common ground in their dysfunctional family lives. To seal their friendship, Mona introduces Tamsin to her born-again Christian brother (Paddy Considine), and Mona helps Tamsin spy on her adulterous father. Bound together by their secrets, Tamsin and Mona see their friendship deepen and enter into dangerous waters.