"A big brother must always stay ahead of his younger brother. That is my responsibility as the older sibling"—these are the words that have always motivated Mutta Nanba to try his best when it came to his younger brother Hibito. So naturally, in the summer of 2006 when the Nanba brothers witness something strange in the night sky, Hibito declared that he would become an astronaut and travel to the Moon, while Mutta countered with the affirmation that he would go to Mars. That was the promise they made to each other.

Now in 2025, while Hibito is working with NASA and preparing to become the first Japanese to land on the moon, Mutta has been fired from his job in an automotive development company. Having difficulty finding another job, Mutta is given a lifetime opportunity when he receives a letter from JAXA—the Japan Aerospace Exploration Agency—telling him that he's been accepted to participate in the next astronaut selection. As if fate has given him a chance to fulfill the promise he made all those years ago, Mutta undertakes the difficult challenge of going to space.