Bai Jie explains to Jiu Shi that they are in a game realm, where the goal is to leave the game level by finding the Door and Key. They must defeat the Ghost to complete the level with other players, where some are newcomers like Jiu Shi, while others have entered other levels before. In the game realm, if a person dies, they die in real life. They must follow certain conditions so as not to be killed by the Ghost. (Source: frankly at MyDramaList)