In the near future, a sudden and unexplained proliferation of plants has left much of human civilization drowning in greenery.
Yuuto is a boy living in a once-thriving city that's now largely reclaimed by nature.
One day, a girl named Kanade appears on his doorstep.
The two had been exchanging letters after Kanade found one of Yuuto's carrier pigeons injured and nursed it back to health.
Kanade then reveals her biggest secrets, too important to write in a letter:
The fact that she's half alien, half human, and that she's on a mission to save the world with her song.
She sings before the bewildered Yuuto, demonstrating her power to pacify the plants.

"I have to sing the greatest love song and save the world in my mother's place.
Yuuto-san, would you please fall in love with me?"

After the disappearance of her mother,
Kanade was left clueless as to how to save the world, or why she had to fall in love in order to do so.
Everything seemed shrouded in mystery.
Still, Kanade's earnest wish stirs something within Yuuto,
who is always ready to lend a helping hand to those in need, and he decides to take a chance on her.
Warmly watched over by those around them, the two tentatively begin their exploration of love.
What neither of them realize is that a spark of affection has already budded between them...

This is the tale of a wondrous future where the encounter between a boy and girl brings a miracle to the world.

[From official website]