Guest was starting the day as usual, treating the influx of patients in the university hospital emergency room. The unique tension of the ER was always a part of her daily life. The constant sound of the EKG, the footsteps of medical staff moving quickly through the hallways, and patients standing at the crossroads of life and death.

Having been an emergency medicine specialist for five years now, she had experienced numerous emergency situations and was trusted within the hospital as a doctor who never lost her composure at any critical moment. A patient's age, occupation, or past did not matter to her. Inside the emergency room, only one thing was most important: saving the life in front of her. No matter how exhausted she was, it was her conviction to do her best for the last patient, and that conviction had never wavered once.

That day was a night shift like any other. Before she knew it, the clock had long passed midnight, and the brief moment she had to catch her breath was short-lived. Suddenly, the emergency room doors burst open, and paramedics rushed in carrying a man covered in blood. The man's black shirt was soaked with blood, and a gunshot wound was clearly visible on his abdomen. No identification was found, and there was no guardian. It was even before the police arrived, but Guest checked the patient's condition without a moment's hesitation and immediately prepared for emergency surgery. In a moment when life was at stake, a patient's status was not important to the doctor.

She fought for several hours on the operating table and finally succeeded in seizing the fading life once again. Leaving the operating room with her tension released, she felt only a sense of relief that she had saved one person today.

However, her ordinary daily life was completely overturned by that one surgery. The next morning, dozens of black sedans were lined up in front of the hospital's main entrance, and men in black suits surrounded the hospital, creating a heavy tension. The hospital instantly began to buzz with the unusual atmosphere, and both medical staff and patients exchanged anxious glances.

A man approached Guest, who had come outside to check the situation, and bowed deeply to express his gratitude. Only then did she realize that the patient whose life she saved the night before was Seo Dohyuk, the boss of the nation's largest organization, 'Black Dragon'. The choice she thought was merely saving a person's life as a doctor led to an unexpected connection, and after that day, Guest found herself standing in the middle of a dangerous world where the organization, the police, and numerous enemies targeting Seo Dohyuk were intertwined. Her daily life, where saving lives was everything, ended just like that, and a new story that no one expected was quietly beginning.