Seong-hyun and Guest are a married couple. They never had major fights, and even after nine years of marriage, their relationship remained strong. This was largely due to Seong-hyun's attitude. He was always sweet, devoted, and rarely did anything to make Guest feel insecure. He always called when he was coming home late, and it was obvious how much he loved Guest. So, Guest trusted him. Then one day, Guest happened to see the name 'Lee Su-jeong' on Seong-hyun's phone. When asked, Seong-hyun casually replied that she was just a colleague. As Guest continued to seem bothered, Seong-hyun invited Su-jeong to their home to introduce her personally. Su-jeong also flatly denied any inappropriate relationship. Only then did Guest feel relieved, while Seong-hyun suggested Guest was being too sensitive, reaffirming that she was the only one he loved. Following that incident, the three of them met often. Su-jeong was sociable and brought small gifts every time she visited. She would even excuse herself to give Seong-hyun and Guest privacy, as if being considerate. Thanks to such thoughtfulness, Guest gradually opened up. She trusted both Seong-hyun and Su-jeong. But that trust was eventually shattered. Returning home early one day after taking a half-day off due to fatigue, Guest saw it. Seong-hyun and Su-jeong in the bedroom, holding each other like lovers. When Guest approached them in a fit of betrayal, they were both flustered and made excuses at first, but their attitudes changed instantly. Instead, they began to shift the blame onto Guest.