Star Ocean is a role-playing video game that is played from a top-down perspective. The player navigates a character throughout the game world, exploring areas and interacting with non-player characters.In progressing through the game, the player is able to recruit up to eight additional characters to travel alongside the protagonist; some recruitments happen automatically, while others only happen depending on the previous actions and options taken by the player. Though only 8 characters can be recruited in a playthrough, ten characters are available to be recruited, meaning that not all characters can be recruited in one playthrough of the game. Most of the game occurs in towns and dungeons, and unlike many RPGs, the game lacks a world map that connects various locations, instead having the character follow paths from one place to the next.
The game starts off in a small town of Kratus on the under-developed planet of Roak. There, a few of the local Fellpool (cat-like people) youth, Roddick, Millie, and Dorne, are part of the village’s local “Defense Force”, who defend the village from minor threats such as thieves and robbers. However, one day, a neighboring town, Coule, starts contracting a terrible disease that turns people into stone. The town healer, Millie’s father, contracts the disease while trying to get rid of it, leading the group to search Mt. Metorx for a herb that is rumored to cure any sickness. Dorne unintentionally contracts the disease as well after touching an infected pigeon.
When they reach the summit, they are confronted by Ronyx J. Kenny and Ilia Silvestri, two crew members of the Earth Federation starship Calnus. They inform them that the disease was sent to the planet by a foreign race called the Lezonians, whom the Earth Federation been at war with. Roddick and Millie go with them on their spacecraft to help them find a cure. They learn that Fellpool blood could be used to process a special, invisible material which could give them a massive advantage in the war. Upon coming in contact with Lezonians, they reveal that they were being forced into war by a shadowy, powerful third party with a disgust for the Federation.
Before Dorne fully succumbs to the disease, they do tests on him to figure out a cure. They determined that the only possible way to fight it would be to make a vaccine that uses the original source of the disease. While the origin of the virus is tracked back to being on Roak itself, it is from Asmodeus, the King of the Demon World, who had been killed 300 years prior to the spread of the disease. Ronyx talks the group into using a Time Gate on the Planey Styx to go back 300 years into the past to track down Asmodeus back when he was still alive.While this works, Ilia trips while approaching the gate. As such, Ilia and Roddick have a delay from when they enter the time gate, and after the trip through time, they find themselves separated from Ronyx and Millie. The two groups work towards locating each other, and Asmodeus, in efforts to heal their family members and stop the war.