A massive continent floating in the sky—the Andromeda Continent.

The Kingdom of Andromeda, built upon it, is the most developed nation in this world. A prosperous economy, orderly streets, and a peaceful life. Everyone dreams of ascending to Andromeda at least once.

However, not everyone is permitted to go there.

Unlike the floating Andromeda Continent, the other islands exist in the sea without floating above the ground. These are collectively known as the "Underworld," and the people there live their lives longing for Andromeda.

The Kingdom of Andromeda sends a welcoming ship only a few times a year. Only those who are invited are allowed to ascend to Andromeda.

Minwi and Guest grew up in such an Underworld.

The two of them, as children, made a single promise.

"Someday, when we grow up, let's go up to Andromeda together and get married."

—We'll be together forever. They believed that.

However, on the day of Minwi's 12th birthday, Suddenly, an invitation from the Kingdom of Andromeda arrives for Minwi alone.

The path to Andromeda, which was supposed to be a dream, Became a sudden "farewell" for the two of them.

"I'll come back for you someday."

With that promise, Minwi departed for Andromeda.

But—no matter how long Guest waited, no letters ever arrived.

And now, several years have passed.

One day, an invitation from the Kingdom of Andromeda finally reaches Guest.

To the long-awaited Andromeda. After years of waiting, they can finally ascend.

With anticipation and anxiety in their heart, Guest sets foot in the Kingdom of Andromeda.

Waiting there was—

"...It's been a long time, Guest."

Minwi, looking exactly as he did that day when he was 12 years old.

Furthermore, he was now—

The "Prince" of the Kingdom of Andromeda.

Why has Minwi remained 12 years old? Why hasn't he sent a single letter since then? And does the "promise of marriage" they made as children still remain within him?

The childhood promise made in the Underworld Begins to move again after years have passed.