In a shrine south of Granseal Castle, a thief named Slade unwittingly breaks the power of a magic seal by stealing the jewels of Light and Darkness; this unseals the demon king Zeon trapped therein. The following day, King Granseal falls sick when he is attacked by an unknown creature and Sir Astral, the court's advisor, along with his apprentices, Bowie the squire, Sarah the healer and Chester the knight, investigate these events at the castle. They find that the door to the Tower of Ancients has been unlocked and stands open.

Greeting their arrival in the tower are devils named "Gizmos" springing forth to attack, though their ultimate intentions are unknown. Upon their defeat, the Gizmo that appears to be the leader flees. It is discovered thereafter to have possessed King Granseal. Sir Astral and the nascent Shining Force visit the king, finding him possessed. Sir Astral exorcises the demon, which promptly flees. The Minister, in place of King Granseal who is looking over a fainted Astral, then orders the slaying of the Gizmo by a platoon of Granseal's soldiers.

When the true danger of the opened door is realized, the Shining Force must fight against the invading armies of Zeon to find the Holy Sword and the stolen jewels so that they may once again defeat and reseal the Devil King Zeon in Arc Valley.