Yukari Morita is a high school student searching for her missing father. During her search in the Solomon Islands, she receives an offer of a lifetime: she will get the help she needs to find her father, and all she needs to do in return is become the world's youngest and lightest astronaut. Yukari and her half-sister Matsuri, who is also petite, are the perfect crew for the Solomon Space Association's launches, or will be once they complete their rigorous and sometimes dangerous training.