Aki Shiraishi is a high school student working in the astronomy club and one of the few witnesses to an amazing event - someone is building a tower on the planet Mercury. Soon, the Builders have constructed a ring around the sun, threatening the ecology of Earth with an immense shadow. Aki is inspired to pursue a career in science, and the truth. She must determine the purpose of the ring and the plans of its creators, as the survival of both species - humanity and the alien Builders - hangs in the balance.