Ruin: Kami no Isan is an action RPG. The player navigates Jan through top-down towns and hostile areas; the "world map" is not free-roaming, allowing only linear advancement from location to location. Hostile areas are populated by enemies who are visibly walking on screen. Unlike most Japanese RPGs, there are no separate "battle screens" in the game; the monsters are fought on the same spot they are encountered on, like in action games. However, unlike most action RPGs, there is a party in Ruin. The player controls only Jan, while the other party members act according to AI routines: attacking, casting offensive and healing spells, etc. The rest of the gameplay is traditional Japanese RPG style: acquiring better weapons and armor, automatic leveling up, linear story development, etc.

Long time ago, a thriving civilization blessed by the gods was ruined as a result of a perpetual war. Humanity has built a new civilization, which resembles a medieval world. The young swordsman Jan lives In the kingdom of Cordelia. One fine day he and Ortena, the daughter of the High Priest, go on a treasure hunt. When they return, they find the country overrun by monsters; Ortena's father is killed, and the princess of Cordelia has been kidnapped. The two team up with a legendary hero, in order to rescue those who can still be rescued, and fulfill the legacy of the gods...