One hot, midsummer's day, Shun Nanami's sister drowned to death. It's a memory he will never forget, which has become lodged forevermore in his heart. He decides from then on to live in her stead, to make up for the life she never got to lead.
Time passes, his agonizing childhood behind him, and he currently spends his days attending school like always. Then, another sultry summer comes around, reminding him ever-so-slightly of his sister once again...
Just then, he hears a piano's melody far off in the distance--a tune she always adored. He follows this music to its source, only to find his sister standing there.
"Please take care of me from now on, Onii-chan."
His little sister passed away long ago, but now, an android who looks exactly like her has appeared before him, "TRINO."