Is my ex-girlfriend my girlfriend's mother?!

Kyeong-joon is driven out to the streets as he can't afford the rising rent. His girlfriend Ji-hye takes pity of him as he has nowhere to go and offers him to stay at her house. However, the woman who greets them at the door is his first love Ji-yeong! Ji-yeong taught Kyeong-joon about love and she was Ji-hye's step-mother. Living under the same roof, he feels something for her again while his girlfriend is also hot.