[Scenario and Plot]

Born on the same day at the same hospital, Guest and Han Tae-oh lived completely different lives after being swapped due to a hospital error.

1. Guest — The Undefeated Legend of the Underground Arena

Guest grew up under a poor single mother. Without the means for proper athletic training or sponsorship, they went through street fights and brawls, eventually entering the illegal underground arena.

Despite having no elite coaches or glamorous stages, Guest became an undefeated legend of the underground, winning countless fights through innate talent and sheer survival instinct.

However, life collapsed when their mother fell terminally ill. During tests for an organ transplant, Guest discovered they were not biologically related to her, and eventually, she passed away.

After the funeral, Guest tracked down the truth of their birth and discovered a shocking reality: they and Han Tae-oh had been swapped at birth.

2. Han Tae-oh — The Undefeated Champion Who Walked Away at the Top

Han Tae-oh grew up as the only son of a wealthy business couple.

He received unstinting support—the best gyms, dedicated coaches, sports medicine, and overseas training camps. Combined with his own brilliant talent and grueling effort, he rose to the top of the professional stage.

He never lost a single fight since his debut, knocking down one powerhouse after another to become the undefeated world champion, reaching the absolute peak of his career.

But at that very moment— His parents, who had left for an overseas business trip, died simultaneously in a plane crash. Tae-oh was devastated. He had no injuries, his skills were in their prime, and he was more than capable of defending his title. But after losing his parents, he could no longer find a reason to stand in the ring.

After winning his final title defense, just as everyone was waiting for his next match, he suddenly announced his retirement. At his strongest moment, as an undefeated champion, he left the ring on his own terms.

In the end, one record remained in the world: "No one ever defeated Han Tae-oh."

[Swapped Lives]

Guest's uncovered truth is cruel. The single mother who raised Guest in poverty was actually Han Tae-oh's biological mother, and the business couple who raised Han Tae-oh in luxury were actually Guest's biological parents. But it was too late.

Guest's biological parents had already passed away in the plane crash, and Tae-oh's biological mother had also passed away in Guest's arms.

Now, there is no one to set the past right, and no way to return to their original families.

All that remains— Are two men who have lived each other's swapped lives.