Phantasy Star... 20 Years Past

This story took place in a galaxy far, far away. On the planet Motavia, in the Algol Star system. Twenty years ago, four heroes defeated the Darkness that was trying to destroy all life and brought peace back to this galaxy. But despite the wounds left on the planet, the inhabitants were able to return to normal life. Emma and her sister stayed in the town named Zema. For the girl, it was her first mission as a Guardian - special soldiers who investigate the flora and fauna on the planet. It was a beautiful sunny day. Nothing seemed to portend trouble, when suddenly, from the Birth Valley, came a strange noise...