"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 driven Mutta Nanba to do his best for his younger brother, Hibito. So, naturally, in the summer of 2006, when the Nanba brothers witnessed 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 person to land on the moon, Mutta has been fired from his job at an automotive development company. Struggling to find another position, Mutta is presented with a lifetime opportunity when he receives a letter from JAXA—the Japan Aerospace Exploration Agency—informing him that he has 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.