In a near-future Japan, the country's first manned spaceflight mission ends in catastrophe. The rocket known as the Lion explodes shortly after liftoff, crashing into the coastal city of Yuigahama and causing widespread civilian casualties. Among the survivors is a one-year-old girl named Asumi Kamogawa, whose mother is left in a coma from which she never wakes. Years later, as Japan attempts to revive its space program, a determined Asumi carries the memory of the disaster and her lost mother as she pursues an impossible dream: to become an astronaut.

The story follows Asumi as she enters the newly established Tokyo Space Academy, a vocational high school created to train the next generation of space explorers. She is among the first students selected for the rigorous astronaut training course, a program designed to push young candidates to their physical and mental limits. Despite her small stature and lingering trauma from the Lion accident, Asumi's unwavering passion for the stars sets her apart. At the academy, she forms a tight-knit group of friends who share her ambition. These include Shinnosuke Fuchuya, her loyal and teasing childhood friend who secretly harbors romantic feelings for her; Shu Suzuki, the easygoing yet highly capable top student who carries a hidden medical condition; Kei Oumi, an outgoing and fiercely loyal girl who becomes Asumi's first friend at the entrance exam; and Marika Ukita, a cold and distant loner who eventually reveals herself to be a human clone, created to replace a deceased original despite an international ban on the practice.

Throughout their training, the candidates face a series of grueling trials, from survival tests and zero-gravity simulations to high-altitude pressure chamber ordeals. The academy's instructors are not always supportive, with one teacher, Takahito Sano, holding a grudge against Asumi because he falsely believes her father, a designer of the ill-fated Lion rocket, bears responsibility for the disaster. As the group grows closer, they make a solemn promise to complete the program together. However, tragedy strikes when Shu is selected for an exchange program with the American space agency. He dies from a hereditary lung condition, coughing up blood, leaving his friends devastated. Kei, who had hidden her feelings for him, is particularly shattered.

The live-action adaptation notably removes the supernatural element of the ghostly astronaut Mr. Lion, who serves as Asumi's spirit advisor in the original manga and anime. Instead, the drama focuses on the realistic struggles of its young protagonists. The story explores themes of grief, national guilt over the Lion disaster, and the sacrifices required to reach for the heavens. As the series builds toward its conclusion, the remaining candidates learn that one among them will be chosen for Japan's second manned mission, a flight intended to complete a space-based solar power satellite. After three years of intense training, Asumi is ultimately selected, becoming at eighteen the youngest person ever to go into space. In a poignant final act, she carries Shu's ashes with her on the journey, fulfilling a promise made under the stars as the rocket lifts off toward the binary star Spica.