One day, a black pillar appeared in the Sea of Japan — a gate that led to another world, used by monsters to invade this world. Dr. Hayakawa, a professor famous for his research in the field of geology, got sent to the other world with his wife Marie. They returned safely, and wrote the book: The Day of Conjunction. Soon thereafter, they left for the other world again. Years later, their now 12-year-old twins Yuu and Ai explore a closed-down subway station, where according to "The Day of Conjunction", a train that travels to the other world departs. To search for their parents, they board the train. Following Ai and Yuu, the 22-year-old Lisa also boards the train. But who is this mysterious woman? And what awaits the twins in the other world?