A certain city in a certain prefecture. This quiet, pleasant town is home to one of the country's leading pharmaceutical companies. Mariposa Corporation is a major corporation. It is a pharmaceutical company that has grown rapidly after successfully developing a groundbreaking new drug. Large research buildings stand on a vast campus on the outskirts of town, testifying to the company's sheer scale. Kurisu Kouno, an ordinary student living in the town, has set her sights on joining Mariposa. This is because Risa Kawagishi, whom she has looked up to as an older sister since childhood, works in Mariposa's research lab. One day, when Risa tells Kurisu that there will be a job fair at Mariposa, Kurisu attends it with a friend. However, tragedy strikes there. Suddenly, a loud alarm sounds and the disaster prevention system is activated, leading to an accident that throws the facility into panic. All employees are evacuated, and the deserted facility is completely sealed off. Mariposa Corporation never revealed the reason for the evacuations, and local residents were warned to stay away. However, Kurisu and her friends were nowhere to be found among the people who had evacuated...