The hero wakes up in the middle of a forest. His head hurts. He doesn't remember anything: he doesn't know who he is, and can only vaguely recall his own name. A man named Darus approaches the hero, takes him to a nearby village and introduces himself as a sorcerer on a quest for revenge. The two join forces, and soon become involved in a complex mystery concerning a kidnapped princess, the destruction of the world, and the search for the hero's own identity.

Radia Senki is set in a semi-modern, semi-medieval environment. It is built like a traditional Japanese-style role-playing game; however, combat is action-oriented. During the pre-set (not random) battles, the player-controlled party and the enemies can freely move around the battle screen. The player controls the main character by attacking in real time, but is also able to pause at any time and give a command to other party members: attack a specified enemy, be offensive in general, use a healing or other technique, pretend to be dead, etc.