The year 2006. Two brothers, Nanba Mutta (age 12) and Hibito (age 9), happen to spot a UFO headed for the moon. The experience inspires them to make a certain promise.

With his mother's assistance and Hibito's encouragement, Mutta has been accepted for the New Astronaut Selection Exam. However, unsure of his next step, he visits Sharon, who works at a nearby observatory. After receiving advice from Sharon, Mutta is finally able to make up his mind.

After advancing to the second exam, Mutta realizes that the other applicants are much better prepared than he is. Desperate to prove himself, he stakes everything on the lung capacity test, as he has confidence in his trumpet-trained lungs. However, as the other applicants drop out, he finds himself in a heated battle with Itou Serika. Who will come out on top?

After a long week, only the final interview remains in the second JAXA exam. Mutta is drawing attention as Nanba Hibito's older brother, but he insists he won't spend his entire life as Nanba Hibito's big brother.

After completing the second JAXA exam, Hibito invites Mutta for a free trip to Houston, Texas, through NASA's family support program. Upon arrival, he finds himself chased by a pug named Apo, and is then reunited with Hibito. However, Hibito expresses disappointment.

Mutta has arrived in America, but he's off to a rocky start with Hibito. Meanwhile, his future as an astronaut is in jeopardy because the headbutt incident has been brought to JAXA's attention.

After learning that his former boss has told JAXA about the headbutt incident, Mutta has given up hope on passing the exam. To cope, Mutta falls back on his childhood habit of multitasking like crazy when something bad happens.

When the robber known as the fire extinguisher man strikes the restaurant where Mutta is, he's in trouble. But once the dust settles, Mutta has become a national hero for capturing the criminal. However, it appears there's more to the story.

After passing the second JAXA exam, Mutta returns to Japan for the third. He's happy about the exam result, but on the flight back, a nightmare leaves him rattled. This is due to the letters from Hibito that he discovered before leaving.

Mutta has learned that Kenji and Serika have strong reasons to become astronauts, while Mutta is simply trying to achieve a childhood dream. This causes Mutta to begin to question himself.

The third exam will take two weeks. Mutta and the other candidates are transported via sealed bus to a large, barren facility. Then they are led to an area that is known as the last room.

The fifteen remaining candidates have been divided into three teams of five. Each team will spend two weeks inside a sealed box, while completing various tasks. The first task is to determine the time.

The sealed box teams are assigned more tasks. These include Running Math, five minutes of mental math while running, and a typing test. Next, they're asked to write a letter of protest to a journalist who is criticizing government funding for space exploration.

After five days in their sealed boxes, each team is exhibiting a pronounced personality. Mutta's Team A is rowdy, while Team C is quiet. However, Kenji's Team B appears to be a little gloomy.

The exam in the sealed box is halfway over. Mutta's Team A suffered an accident when Fukuda's glasses were broken. But Furuya, the one who broke them, requested a new pair. Meanwhile, Team B must deal with an alarm going off in the middle of the night.

The third exam is halfway over. Mutta works on a completely white puzzle as he ponders why Fukuda-san would break the only clock in the sealed box.

The third exam inside the sealed boxes has been underway for ten days. Each team has been experiencing unexpected problems, such as broken clocks and mysterious alarms that ring in the middle of the night. However, Mutta soon realizes who is actually responsible.

The candidates have only a few days left in the sealed boxes. JAXA has been using green cards to create trouble, gauging each member's ability to respond to stress and trouble.

The teams have two days left in the sealed boxes. Mission Control has deliberately not given certain members of Kenji's Team B any green cards, leading to paranoia. Meanwhile, in Team A, Mutta announces that he has good news, bad news, and dicey news.

It's their final day in the sealed box. Mutta has enjoyed his two weeks with the team, and he wishes to avert ruining their bond, as they make their final decision.