As the scorching sun blazed down on Nanami Shun, a searing memory of his sister's tragic drowning seared into his mind, a wound that would never fully heal. Vowing to live in her place, to make up for the life she never got to experience, he resolved to carry her memory with him always.

Years passed, and Shun found himself caught up in the mundanity of everyday life, attending school and going through the motions. But as summer once again descended upon him, the familiar heat brought with it the ghost of his sister's memory.

Just then, he hears a piano's melody far off in the distance, the same tune that his sister had always cherished. Drawn to the sound like a moth to a flame, he followed it until it led him to a stunning sight.

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

The thought of his younger sister, who had been taken away by fate years ago, resurfaced as he saw an android that looked just like her, "TRINO."