The protagonist, Shoko, is a middle school student who goes to Ryusei Academy. Her older sister, who she was very close to, had gone to study overseas 5 years ago and hadn’t been heard from since. One day, something unknown attacks Shoko. The thing that attacked her was a Dryad, a servant of the evil goddess Eris, who had been sealed away by the goddess Athena.

It was then that Shoko learns about Athena, Saints, and Saintias, as well as the cruel fate that awaited her sister.

[Source: Crunchyroll]