Here is the overall storyline of the series. Thanks to animedaisuki for providing information.
One day, the black pillar appeared suddenly and mysteriously in the Sea of Japan. That is a gate which led to the 'another world', and monsters have invaded into our world from there. Dr.Hayakawa, who was famous for the research of the crust energy, went to another world with his wife Marie, and they had come back safely. And, they wrote the book " Kairetsu no hi ( Day of the field row ) ". After that, they never came back though they went to another world again.
Their twins' child Yu and Ai went to the station of subway which had been closed. According to " Day of the field row ", the train which goes to the another world comes at midnight. To search for parents, they tried to take this train. At night of the full moon, the train for the another world arrived at 0:13:13. Yu and Ai boarded the train. And as follow Yu and Ai, the girl named Risa also took the train. Who is she?