I/O takes place in the year 2032. A lunar eclipse occurs in late April, causing a surge of terrorist activities and bizarre incidents. As time goes on, things get worse and worse, with the key to the mystery seemed to be locked away in the MMORPG "Babylon". Four people, through their own ways, investigate the ongoing phenomenon, each one dealing with his or her own problems... The game was originally released on the Play Station 2 in 2006, but was given an Updated Rerelease on the PC two years later, with new CGs and slightly updated artwork.