A futuristic Beijing is divided into three cities based on class. First-class people live in the first space; second-class people live in the second space; and third-class people live in the third space. Each class takes up space for a specified time before the city folds and replaces it with the new class. When a city folds, the occupants are put to sleep.  Lao Dao is a third-class waste worker. He has a daughter and needs to raise money to send her to kindergarten. Qin Tian, a student from the second space, offers to give him money if he delivers a love letter to Yi Yan, Tian’s lover in the first space. Traveling among the spaces is illegal, and Lao Dao could be imprisoned if he is caught. However, he agrees to deliver the letter. Lao Dao manages to arrive at the first space with the love letter but discovers that Yi Yan is married. Yi Yan then bribes him not to tell Qin Tian about her marriage. Desperate for money, Lao Dao agrees. As Lao Dao returns to tell Qin Tian that he gave the letter to Yi Yan, a security official captures him while Lao Dao is in second space. The official doesn’t send him to prison for traveling between the spaces, as he was born in the third space like Lao Dao. The official reveals that the central economic pillar of the third space, the waste processing industry, could easily be replaced by technology, and it is only kept to ensure that workers have jobs. He also reveals that the city folds to force workers to sleep, a situation that causes workers to have fewer hours to ensure that there is more work for people in the other spaces. After telling Qin Tian that he completed the mission and doesn’t reveal that his love interest is married, Lao Dao returns back to the third space with the money and continues going to work.