Tsubasa Shiina, a ten-year-old girl, has just moved to Hokkaido to live with her father. At school, she is withdrawn and keeps to herself. One night, she sees a UFO crash in the woods near her home. Rushing to the scene, she discovers the pilot, codenamed "D.D.," in critical condition. Also present is a Maguar, a grotesque alien that hatched aboard D.D.'s ship. As the Maguar is about to kill Tsubasa, an alien lifeform merges with her body, creating the Riberus battle armor Figure 17, which swiftly destroys the alien. The lifeform takes the name Hikaru and becomes Tsubasa's identical twin. Over time, Tsubasa grows more outgoing thanks to her new sister. However, with Maguar eggs scattered across Hokkaido, they must continue to form Figure 17 to eliminate any hatched Maguars until backup from D.D.'s home planet arrives.