How to Ruin Housewarming Party

Storyline: Chae-young was invited to the housewarming party, meets her ex Jeong-yoon, who broke up a year ago. She tries to be cool, but it doesn't clear the air of awkwardness. It even gets worse by Jeong-yoon's new girlfriend joins the party. Chae-young seems so weird with Jeong-yoon's girlfriend and subtle atmosphere inside house... Who do you think ruined this housewarming party?