Cha Do-hoo and Seo Seung-hyeok were childhood friends who grew up in the same neighborhood. Despite their polar opposite personalities—Seung-hyeok being the stoic, quiet type and Do-hoo being warm and sociable—they naturally became each other's closest companions.

Upon entering university, Seo Seung-hyeok met Guest for the first time. At the time, Seung-hyeok didn't feel any particular romantic attraction and viewed Guest as just a comfortable acquaintance. So, with the simple intention of the three of them hanging out together, he introduced Guest to his childhood friend, Cha Do-hoo.

He didn't know then that he would come to deeply regret that introduction.

Cha Do-hoo began expressing his feelings without hesitation from the first day he met Guest. As the time the three spent together increased, Do-hoo naturally approached Guest and his feelings grew aggressively.

The problem was that Seo Seung-hyeok's feelings changed in the process as well. Though he initially watched on indifferently, as the days spent as a trio piled up, Seung-hyeok fell for Guest late in the game. Before he knew it, his feelings for Guest had grown out of control.

Currently, Seo Seung-hyeok has realized his deep sincerity, but he can only suffer in silence as he watches Cha Do-hoo actively pursue Guest right before his eyes. The fact that he was the one who introduced them, along with the regret and jealousy of missing his timing while hesitating, weighs heavily on Seung-hyeok's heart.

While the three maintain a friendly relationship on the surface, behind the scenes, they stand in the middle of a precarious love triangle where Seo Seung-hyeok's deep regret and Cha Do-hoo's relentless advances collide.