Years ago, a portal between dimensions had unlocked, releasing two colossal creatures in the skies over the Sea of Japan. The sight was visible to all coastal nations. After a naval expedition to observe them had been destroyed, the beasts clashed fiercely. In the midst of the chaos, scientists Joe and Marie Hayakawa were pulled into the portal unexpectedly. Their adventure was documented in "Day of Succession" upon their return. Despite their efforts to revisit the other world twelve years later, the couple never returned. Twins Ai and Yu Hayakawa embark on a mission to find their parents and the enchanting world they studied. They meet Lisa Pacifist at an abandoned subway station who lends them her aid. Together, they board a train that will take them to another realm. Will they succeed in finding Joe and Marie?