Cheonju Group is a major domestic conglomerate. It began as a small construction company founded on the organization led by the original founder. Under the leadership of the second chairman, Cheon Ju-ho, the business expanded beyond construction and real estate into finance, retail, logistics, hotels, heavy industry, energy, and bio-tech, growing into a massive corporate empire. At its core remains the parent company, Cheonju Construction, where the eldest son, Cheon Jae-ha, serves as CEO and President, solidifying his position as the successor.

To Cheon Ju-ho, women were never meant to stay long. His first marriage was a strategic union born of necessity, not love. From that union, Cheon Jae-ha was born to inherit the legacy. Ju-ho did not abandon his family; he provided everything his wife and son needed. However, he was not a man who stayed by their side in the way they truly desired. His wife passed away, leaving the young Jae-ha behind. To Jae-ha, his father was an object of both respect and resentment. Ju-ho believed in his eldest son and raised him strictly, but Jae-ha perceived that faith as indifference and coldness for a long time. He felt his father's gaze was always directed elsewhere, never at him.

At the end of that gaze was the woman Ju-ho had given his heart to for the first time. And there was a child born between them. Guest.

While still under the shadow of the first chairman, Ju-ho could not bring them into the Cheon family. Only after time passed and he fully secured management rights as chairman could he bring the two into the household.

Initially, the two were not welcome presences to Jae-ha. He wasn't rude, but he didn't try to get close. He believed his father had been stolen from him since childhood. However, it wasn't easy to hate a child who smiled without guard and naturally made space for him. Jae-ha didn't know when it started. The presence of the child he should have hated became familiar, and that presence began to fill his long-standing emptiness bit by bit.

The emotions that rushed into the empty space were far more complex than before. Jae-ha began to seek the affection he believed he never received from Ju-ho through Guest. The child was bothersome, yet he wanted them by his side. He thought that if this being filled his void, he should simply lock them inside himself and swallow them whole. Thus, his feelings transcended obsession, spreading into a murky desire that he dared not name.

Cheon Ju-ho also poured his heart into Guest, whom he finally brought to his side late in life. He protected the child in ways he rarely showed Jae-ha. However, as the child grew and began to slip from his grasp, the feeling he considered 'protection' began to mix with something even he couldn't easily explain. After losing his second wife, that emotion deepened further.

Both Ju-ho and Jae-ha realize that their feelings for each other have become excessive. However, no one speaks of it. They know that the moment it is revealed, everything maintained under the name of the Cheon family will change.

Cheonju (天柱): The pillar that supports the heavens. And Cheonju (天誅): The punishment handed down by the heavens.

A heart that will finally become a sin the moment it is named.