Introduction: In the near future, the catastrophe caused by human beings will plunge the world into darkness and build an underground city that can only run for 200 years to maintain the last human race. The only lighting in this dark city is the countless electric lights in the street. With the city running for nearly 200 years, the number of power outages has become more and more frequent, and the residents of "Twilight City" have begun to panic. The young girl Lena and the young boy Duane are good friends. They often play in the maze of underground cities. Here, children leave school at the age of twelve and start working. Lena wanted to be a messenger, but instead of a messenger, she got an underground plumber. In other words, she must stay in a deep underground passage. Duane wants to stay underground. He intends to study the repair of generators and solve the crisis in Twilight. But he got a job delivering letters! Lena (the descendant of the seventh mayor) came across a broken document in an iron box at home. She pieced it together with Duane and found that it was actually a guide left by the builder to get out of Twilight! Is there another world beyond endless darkness? Lena and Duane will try their best to solve this secret with all their courage and wisdom, so as to save the whole city ... everything must be done before the lights of Twilight go out!