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Jun Ji-hyun as Kwon Se-jeong, a biotechnology professor. After losing her professorship due to her intolerance for injustice, she arrives at Nest Building for a conference at her ex-husband Han Gyu-seong's suggestion, trying to land her a new job, and gets trapped inside. As a biotechnologist, she fights to lead survivors and escape, deciphering the infected's behavior and evolution to survive. •
Koo Kyo-hwan as Seo Young-cheol, the biology scientist responsible for the outbreak. A genius biologist, he previously worked at a biotech company, envisioning a new humanity. At the outbreak's center, he becomes a target for authorities and survivors after revealing he has the vaccine. •
Ji Chang-wook as Choi Hyun-seok, a security team member at Dunguri Building, struggling to protect his family during a sudden infection outbreak. Trapped with his older sister Choi Hyun-hee, who was visiting for his long-awaited vacation, Hyun-seok fights to protect her, as she has a lower-body disability. •
Kim Shin-rok as Choi Hyun-hee, an IT company employee and Hyun-seok's older sister, she visited the Nest Building to have lunch with her brother during a long-awaited vacation, ending up stuck with other survivors when the infection broke out. •
Shin Hyun-been as Gong Seol-hee •
Go Soo as Han Gyu-seong, Se-jeong's ex-husband. ==Production==