A lonely girl in the woods finds something broken falling from the sky, and the show never stops being about what it means to care for something that isn't yours. The sci-fi setup dissolves into pure character study: two beings learning to live in the same body, speaking to each other through a journal, figuring out how to share a life neither chose. By the finale you've forgotten there's supposed to be an alien threat at all, because what actually matters is whether Tsubasa and Hikaru can become something like sisters before time runs out.

A quiet ten-year-old girl named Tsubasa has just moved to Hokkaido to live with her father, and at school she keeps to herself. That changes the night a UFO crashes in the forest near her home. At the site, Tsubasa finds the badly hurt alien pilot, known as D.D., along with a monstrous Maguar that hatched aboard the ship. When the creature nearly kills her, an alien lifeform merges with her body to form the battle armor Figure 17 and destroys it. The lifeform then takes the name Hikaru and becomes Tsubasa's identical twin. With her new sister beside her, Tsubasa gradually opens up to the people around her. But Maguar eggs are scattered across Hokkaido, and the two must keep forming Figure 17 to wipe out each creature that hatches until reinforcements from D.D.'s home world reach Earth.