After relocating to Hokkaido with her father, 10-year-old Tsubasa Shiina leads a reserved life at school, distancing herself from peers. One fateful night, she stumbles upon a UFO crash and rescues the injured pilot, known as 'D.D.', along with a menacing alien creature, the Maguar. To save herself, Tsubasa merges with an alien entity, becoming the formidable Riberus battle armor, Figure 17, and vanquishes the alien menace. This fusion births Hikaru, her identical twin, who catalyzes Tsubasa's transformation into a more sociable individual. Tasked with eradicating Maguar threats across Hokkaido using Figure 17, the sisters await reinforcements from D.D.'s alien homeland to safeguard Earth.