One day, a single letter was delivered to a girl who lived on the outskirts of Tokyo and grew up in a fatherless home, named Kurumi Haruka.

It was sent to her from her past, seven-year-ago self.

However, on the letter were words that she didn't understand the meaning of.
"It must be a prank". With that in mind, as she carried on living her "unchanging days", she learned that an identical letter was also sent to each of her four male classmates, Ace, Neji, Kyou and Shuri.

Since receiving the letter, they all got mixed up in various incidents, and were made to face their past, hidden scars.

In doing so, they realised...
...That there just might be something odd about this "world".

What is the truth behind this story that is hidden across the blue sea?