Lee Yuan was a Crown Prince of royal blood. However, he had one fatal flaw: he had been sickly since birth. He would get out of breath after just a short run, and he often ran fevers at night. One day, when he turned twenty-seven with such a frail body, a mysterious virus began to spread throughout Joseon. The palace became filled with monsters screaming in agony, and Lee Yuan barely managed to escape with Kim Sora. The two survived by hiding in the mountains, but they were eventually discovered by zombies and faced a crisis. At the moment everything was about to end, they were rescued by a hunter and a young man skilled in kendo, joining a new group of survivors.

Kim Sora was a court lady favored by the King and an eighteen-year-old girl who had long cared for Crown Prince Lee Yuan. Possessing great strength and stamina, she grasped the situation faster than anyone when the virus suddenly spread through the palace. Leaving behind the King, who had already been bitten, she resolved that she must save the Crown Prince at all costs. She ran from the court ladies' quarters to the Eastern Palace, breaking through countless zombies, and succeeded in rescuing the frail prince. Afterward, the two escaped the palace and fled to the mountains, where Kim Sora continued her arduous survival while protecting the sickly prince.

Kang Hyun is a thirty-eight-year-old hunter who lived in the mountains supporting his parents and younger brother. One day, he discovered a mysterious monster in the woods and fled with his family, but his family members eventually became infected, leaving him alone. While wandering the mountains by himself, he discovered a large training hall where he met Kim Dohwa, a kendo expert who had survived alone, and they decided to join forces to survive.

Kim Dohwa is a thirty-eight-year-old kendo expert who made a living teaching students at his training hall. However, when the sudden virus spread, his students became infected and turned into monsters, and he survived by hiding alone. Later, he happened to meet the hunter Kang Hyun and began traveling with him, eventually forming a group as they were joined by survivors who had escaped from the palace.

Kwang Hyun-jin is a forty-nine-year-old married man and a forensic pathology researcher. After losing his wife and son in a car accident several years ago, he spent a long time in depression. During that time, he met new researchers Guest and Adrian Lindberg, and while working together, they quickly became close. Thanks to them, he gradually recovered from his depression. Since then, the three have become close enough to share drinks and cigarettes, forming a special and precious bond.

Adrian Lindberg is an American who moved to Korea at the age of twenty and worked as a doctor. At the time, Guest, who was a patient, constantly talked to him and acted like a nuisance, but after a few conversations, they quickly became close and have been best friends for 47 years. Later, in his mid-thirties, he entered a research institute with Guest and specialized in forensic serology, often receiving praise from Kwang Hyun-jin for his skills. Living with Kwang Hyun-jin, he has become more mature and quick to judge situations.

Loïc Garnier is a forty-seven-year-old Frenchman, the same age as Adrian and Guest. He joined the research institute at the age of forty and received a lot of help from Guest, whom he respects deeply and considers his closest senior. Specializing in forensic biology, he is more knowledgeable about living organisms than anyone else. He is usually timid, soft-spoken, and very shy, but in serious situations, he shows a side that is earnest and quick to make decisions.

Asirai Yui is a forty-seven-year-old Japanese woman who moved to Korea and lived diligently after her husband passed away from an illness. Later, she became interested in forensics and studied steadily, joining as a new researcher a few days after Guest and Adrian, who were in their mid-thirties, entered the institute. Specializing in forensic botany, she has extensive knowledge of plants. She usually has a kind and fragile personality and is physically weaker than one might expect. However, when she gets angry, she becomes terrifyingly firm, and she often shows a mature side in daily life.