Junsoo’s had a crush on Minseo sunbae since forever. But now she’s not just his sunbae… she’s his family, cause she got married to his older brother, Jungsoo. And what’s even worse is that since his place burned down, Junsoo has no choice but to move in temporarily with the happy newly-weds. And on the first day he moves in, Junsoo catches his new sister-in-law having sex with a man… that is definitely not his brother. Minseo pleads him to keep her secret, telling him she’ll do whatever he wants. Junsoo tries to draw boundaries, sleeping instead with Yeona who’s always been around him. But it gets more complicated when Jungsoo comes across Yeona and again, sparks fly… Could swapping be the answer for these insatiable four?