Cain Morgan is the adopted son of the leader of the Security Force in the kingdom of Heingaria. One day, Cain has a strange dream that brings back memories from his childhood, when his native country Solcia was invaded by the neighboring Aldine. All he remembers is that "everyone went to the island". This mysterious Island of No Return is presently of great interest to the king, who is trying to find why the Elders sent a research team there. The island is rumored to harbor a ring with unseen powers, and the king secretly dispatches Cain to investigate the fate of the people who had ventured there and disappeared without a trace.
Eternal Ring is a first-person action role-playing game similar in concept to King's Field series. It is focused on exploring interconnected dungeons, fighting enemies within. Compared to King's Field games, its world is smaller and less open. With a few exceptions, the only type of weapon Cain can use are swords. However, weapons play a relatively small role in the game, and there is also no armor. The gameplay system largely relies on magic spells that the player can gain by equipping special rings and imbuing them with gems.
The spells and the gems containing them come in six different elemental flavors: Fire, Water, Earth, Wind, Light, and Darkness. The player creates rings by experimenting with different types of gems, resulting in various effects. Up to six gems can be placed on both sides of a ring. Gems also have different levels which affect the outcome of the experiment. The resulting rings may modify Cain's attributes, increase his affinity for a particular element, provide auxiliary bonuses, or grant Cain powerful attack spells.