In the summer of 2006, Mutta Nanba and his younger brother, Hibito, witnessed what they believed was a UFO flying towards the Moon, leading them to vow to become astronauts. Hibito set his sights on the Moon, while Mutta aimed for Mars, convinced he must be one step ahead. As an adult, Mutta finds himself working diligently at an automotive company, while Hibito is on track to become the first Japanese man to step on the Moon. However, after losing his job, Mutta is unexpectedly accepted by JAXA to participate in the next astronaut selection, giving him a chance to catch up to his younger brother. Though reluctant at first, Mutta embarks on an ambitious journey to fulfill the promise he made 19 years ago, despite his self-doubt about his prospects.