Sanmon City, with a population of 280,000, suddenly opened a door to an unknown world one day. From the other side of the door, monsters called "Near-boundary people" swarmed out. Humans panicked and found that the existing weapons on Earth had no effect on the Near-boundary people. In a critical moment, the mysterious group Boundary Defense Organization "BORDER" appeared to repel the Near-boundary people, and Sanmon City finally returned to its normal life in the past. Four and a half years later, a junior high school student Sanyun Xiu (voiced by Kaji Yuki) who was hiding his identity as a member of "BORDER" met a transfer student Kuku Yuma (voiced by Muranaka Chi), and the other party turned out to be a Near-boundary person from the other side of the door. The purpose of this trip was to find his father. The two people from different camps are destined to collide fiercely in the battle of destiny... This film is adapted from the manga of the same name by Ashihara Daisuke.