"Space Brothers" centers on the lives of two brothers, Mutta and Hibito Nanba, who share a lifelong dream of becoming astronauts. The narrative begins in 2006, when the brothers witness a UFO and vow to explore space together. Years later, Hibito achieves his dream and is selected as a candidate for a lunar mission, while Mutta, an automotive engineer, faces professional setbacks. After losing his job, Mutta receives an unexpected opportunity to apply for the Japanese Aerospace Exploration Agency (JAXA) astronaut program, reigniting his childhood ambition.

The series meticulously follows Mutta's journey through the rigorous selection and training process, highlighting the challenges and camaraderie among the candidates. Mutta's determination, creativity, and problem-solving skills are tested as he navigates physical, mental, and emotional hurdles. Meanwhile, Hibito's path as an astronaut is fraught with its own trials, including the pressures of space exploration and the risks inherent in his missions. The brothers' bond remains a central theme, with their mutual support and shared memories serving as a driving force throughout their respective journeys.

The story also delves into the broader context of space exploration, portraying the scientific, technological, and human aspects of the field. Secondary characters, including fellow astronaut candidates, JAXA officials, and family members, contribute to the narrative by providing diverse perspectives on ambition, perseverance, and the sacrifices required to pursue such a demanding career. The series balances personal growth with the grandeur of space exploration, offering a grounded yet aspirational portrayal of the challenges and triumphs faced by those who dare to reach for the stars.