It is summer 1976 and a heatwave is gripping Ireland. Nine-year-old Megan lives in a redbrick house in Dublin with her mother, a beautiful and lonely artist, and her grandmother; her father's whereabouts are a mystery that she often thinks about. When an American family moves in downstairs and Megan's mother begins a tentative affair with the father, everything that Megan is sure of starts to unravel.

From her friendship with the boy next door to the strange food that the new family cooks, from her relationship with the impossibly sophisticated Beth to the possibilities opened up by the temptation of swimming at night, it is a summer that will impact Megan in ways she could never predict.