The series begins with a raid on a group of bandits at the hands of the 15-year-old sorceress Lina Inverse. On her way to Atlas City, she is surrounded by bandits. Just as Lina prepares to attack, a mercenary swordsman, Gourry Gabriev, appears and defeats the bandits. Lina and Gourry befriend each other. Soon after, Lina and Gourry are asked to exterminate the dragon threatening a small village. Having no reason to refuse, Lina takes on the job despite Gourry's dissuasion. Being intimidated by the dragon, Lina is outraged. Lina uses the Dragu Slave technique on the dragon, killing it and destroying most of the village. Lina and Gourry eventually run away from the villagers after the village's destruction.

Lina and Gourry go to a magic shop in town to sell the treasures that Lina snatched from the bandits. However an item from the booty, a knife, is cursed and it turns the shopkeeper violent and a hullabaloo starts. Lina and Gourry fight a hard battle but the chaos is brought to a sudden stop by a single bark of the shopkeeper's wife. While they are relieved, a mummy called Zolf, attacks our heroes with the evil trolls. He is looking for one of the items Lina possesses. Lina and Gourry are fighting their way out. However, on the same evening, Zolf shows up to their room again with a mysterious person.

The mysterious person says that his name is Zelgadis Greywords. He is a mixture of human, golem and demon; a creation of utter evil. He offers to buy Lina's booty, but Lina asks for an outrageous price which drives him away. Not giving up, Zelgadis comes back to take Lina's booty by force. Lina is hurt and thrown into a pinch. Luckily she is saved by the legendary great sage, Rezo the Red Priest. Rezo warns Lina that Zelgadis wants to resurrect the mazoku Shabranigdo, the demon king, then Rezo mysteriously disappears after performing some unknown spell...

Lina and Gourry start investigating a way to stop Shabranigdo's resurrection, however they are plagued with a question. Why is Zelgadis trying to revive the Demon Lord? On the other hand, Rezo cannot easily be trusted. The situation becomes more complicated when Zelgadis and Zolf realize that they are no longer able to scry for the as yet unknown item that is in Lina's possession. While our heroes are traveling through a forest, Lina informs Gourry that she will not be able use her sorcery effectively for a few days. This makes her vulnerable to any villains that may challenge her. As fate would have it, Zelgadis and his men come to attack. During the exchange, one of Zelgadis's men, Dilgear the Werewolf/Troll, lets slip that it is, in fact, the Orihalcon statue that has them chasing Lina across the countryside. Lina narrowly escapes the initial attack and is separated from Gourry. However, she is quickly ambushed, knocked unconscious with a spell, and captured by Zelgadis.

Lina awakens to find that she is held captive in Zelgadis' hideout. After Lina is questioned about the Orihalcon statue, she lies about why Zolf is unable to locate it magically. She is then asked why she didn't fight back harder in the previous skirmish. Zelgadis determines that she should be kept alive until they retrieve the statue in any case and she is then subjected to threats of torture by Zolf and repercussions for her previous treatment of him, much of it ineffective, to his dismay.

Rezo suddenly appears during Lina's escape of and calls Zelgadis a traitor. Lina is surprised to discover the relationship between the two. Zelgadis' grudge against Rezo makes him revolt against him. Lina and Zelgadis barely make it out virtually unharmed, with Zelgadis using Lina as a distraction against Rezo, with quite a bit of protest from Lina. Rezo's men discover the escape, as well as the cooked Noonsa, and subsequently eat him, all while swearing vengeance for his death. Soon after escaping Rezo, Lina and Zelgadis take a break to rest and Lina, being quite confused after find out about Rezo and Zelgadis's relationship, asks Zelgadis to explain to her just what, exactly, is going on.

Under the villains' ceaseless attack, Lina and Zelgadis have no time to rest. Meanwhile, Rezo begins to summon the demons in order to capture them. Outnumbered by the enemies, Lina and Zelgadis get into a pinch. Suddenly a shining knight pops up. It's Gourry who has been looking for Lina. He draws his ancestral sword, the Sword of Light, and slashes the fiends. Zolf and Rodimus, two loyal followers of Zelgadis, come to help them, our heroes shake off Rezo and his band.

While taking a rest in a village, Gourry and Zelgadis start fighting over the Philosopher's Stone. Lina calms them down but they become more hostile with each other. Then, they receive a message from Rezo, stating that unless they give the Philosopher's Stone to him, he will turn all the villagers into stones. Having no choice, Lina and the others go to meet Rezo. Lina has no intention of giving the Stone to Rezo because she knows that the Red Priest is scheming to resurrect Shabranigdo. Rezo manages to hypnotize Zelgadis and makes him steal the Stone. With the Stone in his possession Rezo revives the Dark Lord Shabranigdo.

Rezo succeeds in resurrecting the Dark Lord, Shabranigdo. However, Rezo's body is taken over by the Dark Lord which was not part of his plan. Zolf and Rodimas recklessly challenge Shabranigdo, but are no match to the power of the Dark Lord. All Lina and others can do is to run from the ferocious demon. Shabranigdo's evil power spreads out to the surrounding areas. The weather turns abnormal and monsters become wild. Seeing the monsters attack the village, Lina and others decide to fight back.

Lina and the others challenge the Dark Lord, Shabranigdo. Unfortunately, he is too powerful and no magic works against him. Gourry is knocked down and Zelgadis is struck hard. Then, Lina is given the Sword of Light and it increases the power of her sorcery. Lina challenges Shabranigdo with the most powerful incantation, Giga Slave, which is superior to the Dragu Slave. She realizes that Rezo is still inside Shabranigdo. In the final moment, Rezo gets away from the body of Shabranigdo and finally Lina's Giga Slave smashes the Dark Lord. Upon arriving in Atlas City, Zelgadis decides to part ways with Lina and Gourry.