In a near future where humans have safely traveled to Mars, the space program aims to reach Jupiter for the first time in history. For Subaru Yuuto, Amami Kosei, one of the elite astronauts aboard Jupitonia (the vessel bound for Jupiter), is not only someone he admires but also a close friend of many years. However, tragedy strikes when a malfunction triggers an explosion on Jupitonia, causing the ship to vanish into deep space, with most believing the crew to be dead. Despite the devastating loss of a friend and hero, Yuuto enrolls in the Core Space Academy to become a super-class astronaut (known as a Sirius), just as Kosei once was, and to explore parts of the universe that have not yet been known to man. The competition is cutthroat, and he must rise to the top if he hopes to follow in his hero's footsteps. Although the fear of the unknown is immense, Yuuto must learn to overcome it and become a hero for the next generation.