It has been six years since Rarity had left after being disowned and outcasted by her parents; six years after that "disgraceful" incident with Sweetie Belle.
Sweetie Belle, on the other hoof, greatly endures the struggles of everyday life, painfully looking at masks while waiting her eighteenth birthday: the day which she can leave her home and escape her parents' judgement.
Tonight, however, is different from her usual torture. On this night, every year, she is visited by her lover: the one whom she passes her days for, the one that she would give anything to be with despite what the world tells her, the one she keeps closest in her heart: her beloved sister.