One summer day, Shun Nanami meets a girl who is the spitting image of his dead sister. She turns out to be a high-tech android indistinguishable from a human being. So begins a touching tale in a world where androids and human beings coexist.

One hot, midsummer's day, Shun Nanami's sister, Shirone Nanami, drowned at sea. It's a memory he will never forget; one that will forever be etched into his heart. He vows to live in her stead from then on and to make up for the life she never got to have.

Time passes and Shun currently spends his days attending school like any other student. Yet another sultry summer bears down upon him, reminding him of the sibling he had lost all those years ago...

However, that summer was different. Shun hears a piano's melody far off in the distance--a tune Shirone always adored. He follows the sound to its source, only to find... his sister, Shirone.

"Please take care of me from now on, Onii-chan."

The being that stood before him was not of his own flesh and blood, but an android who looked exactly like her. Her name was...

"TRINO."