The Death of the Heart by Elizabeth Bowen

In this piercing story of innocence betrayed set in the thirties, the orphaned Portia is stranded in the sophisticated and politely treacherous world of her wealthy half-brother's home in London. There she encounters the attractive, carefree cad Eddie. To Portia, Eddie is the only reason to be alive. But when Eddie follows Portia to a sea-side resort, the flash of a cigarette lighter in a darkened cinema illuminates a stunning romantic betrayal.