Mercedes' story starts with her taking her mother's bow into the woods of Ringford to hunt frogs. Elfaria and her nephew Melvin are distressed meanwhile on her lack of awareness of her duties as princess. After, Elfaria was critically wounded in battle with Odin during the Cauldron War and she tells Mercedes in her last moments to be strong, as it would be her duty to be the queen now. Initially Mercedes balks at her new duties overwhelmed, and Melvin steps into her place.
Melvin then offers Mercedes to take a vacation from her new duties, promising to handle her duties for her. Mercedes' guardian Mathew became uncomfortable with Melvin's actions, and suggested Mercedes travel to Elrit Forest to request the Sorcerer Beldor's aid. As she traveled through the forest seeking Beldor out, a frog startled her into dropping her bow into a swamp. The frog revealed it was intelligent and could talk and promised to find it if Mercedes would grant her a wish. While the frog was away searching for it, Beldor arrived, and informed the new queen that he was siding with Melvin. With that he sicked his Dragon servant Belial on Mercedes, though the frog arrived just in time with her Bow. Mercedes battled with Belial and defeated him, forcing Beldor to flee. Belial warned Mercedes that rebellion was brewing in Ringford, causing Mercedes to fly away in fear, leaving the Frog behind.
Mercedes encountered a number of fairy royal loyalists who had traveled to Elrit looking for her. Mathew informed Mercedes that Melvin had taken control of a third of the military and had the support of half the fairy Nobles. A few of the fairy warriors broke free and came looking for Mercedes, though the number she had seemed to few to do anything with. Scared, Mercedes began to panic and ask for somebody, anybody to do something. The frog returned at this point and gave her a few encouraging words, giving Mercedes the strength to take matters into her own hands.
Mercedes led her forces to Ringford, and managed to arrive at the Royal palace, where Melvin ambushed her. He sent his Shadow Knight Oswald to slay the young fairy, though the Halja were at the time preparing to collect Oswald, weakening the Shadow Knight's powers and sapping his strength. Because of this, Mercedes was able to defeat Oswald, and her forces retook Ringford from Melvin, who was wounded in the battle and died. from his injuries.
Following this the Frog revealed his wish to Mercedes: he was actually a human, cursed into the form of a frog by the Sorcerer Urzur, one of Beldor's allies. The curse could only be broken by the kiss of a powerful fairy, and concluding that Mercedes, being royalty, must be a powerful fairy, he requested her to kiss him and break the spell. Being unfold of frogs and uncomfortable with the idea of kissing a potential poisonous and slimy animal, Mercedes stalls on fulfilling her side of the promise.
Though peace returns to Ringford for a short while, it was soon broken by the arrival of the Dragon King Wagner, who came to Ringford seeking revenge for his friend Hindel who had been killed by Oswald previously. Since Oswald was already dead they could not calm Wagner, who ordered the fairies to bring his killer to Winterhorn Ridge or he would return to burn Ringford to the ground. Mercedes traveled to the Dragon's roost to attempt to explain the situation to the dragon. Wagner initially laughed at Mercedes' claims that she had killed Oswald, and when the queen showed her psypher in an attempt to prove herself, Wagner became enraged instead, angered by the sight of a Psypher, the same weapon that killed Hindel.
Wagner attempted to kill Mercedes, though she managed to defeat and calm him. She promised to Wagner she would not do evil with her Psypher. Wagner then demanded just what she would do with it, to which Mercedes replied she would defeat Odin. Wagner laughed at her bravery, but allowed her to leave Winterhorn in peace. Saying that both forces should wipe each other out.
Back at Ringford, Mercedes realized she would need a stronger Psypher to defeat Odin and his Balor. Therefore she needed the aid of the Psyphersmith Brom. Unfortunately Brom had been sent to the Netherworld previously by Melvin and Beldor, and none of the fairies knew the way there. Therefore, Mercedes was left with no course of action other than to request Beldor's aid. She sent a message to him in Titania promising him forgiveness and pardon for his previous actions if he aided her. Mercedes and a small entourage traveled to Titania to meet with Beldor, though the sorcerer did not want her forgiveness, and so Urzur set an ambush for her. Mathew was captured in the ambush and Mercedes was forced to fight her way through the back alleys of Titania which were no swarming with lesser mages and their summoned beasts. Eventually Mercedes faught her way to the Sewers, where Beldor was waiting for her. He again set Belial on the fairy, though the dragon was unwilling to battle Mercedes again. Beldor attempted to force Belial into battle. His abuse of the dragon angered Mercedes, causing her to withdraw her previous offer and present him with a new one: Aid her or die.
Mercedes and Beldor fought, though Beldor managed to trick the princess by attacking her from behind. Before he could succeed, the Frog cast the Pooka Curse upon Beldor, cursing him into the form of a harmless aged pooka. The frog then easily overpowered Beldor. The effort wore the frog out though, and he urged her to escape before the other two wizards arrived. Overcome by pitty for the Frog, Mercedes finally pulled through on her side of her promise, breaking his curse with a kiss. The frog was revealed to be Ingway, who thanked Mercedes and left, saying he had unfinished business, and promising to meet her again.
Afterward, Mercedes forces the captured Beldor to direct her to the Netherworld to save Brom. At the first opportunity, the cursed Sorcerer ditches Mercedes and attempts to escape, fearing the likely results of being found by a Halja while still bound and unable to cast magic anyway. Unfortunately for him, a Halja arrives and reaps him only moments after he abandoned Mercedes. Meanwhile the fairy princess finds Brom, who is being forced to work on Phozon Crystals under the orders of Queen Odette. Mercedes fights Odette in an attempt to save Brom, and manages to fight valiantly for a time, before a horde of Halja arrive to surround her. Before they could finish the princess, a disturbance suddenly distracts the dead. A lesser specter arrives and informs Odette that King Gallon was causing an uproar in his cell. In fact this was because at that moment Ingway was using the Darkova spell in an attempt at revenge on Odin, indirectly saving Mercedes again. Odette tell her Halja to ignore Mercedes, sending them instead to restrain Gallon, giving Mercedes the chance to escape with Brom in tow.
Back at Ringford, Brom eventually agrees to create a psypher powerful enough to counter Balor, but informs Mercedes they would need a fire of unmatched heat to forge Phozon Crystals, and thus they needed a sample of the Source Fire kept in the Fire Kingdom. The pair set off for the Volkenon Lava Caves and manage to make it to the fire's shrine without attracting attention. At that moment, King Valentine arrives and informs the pair that Inferno King Onyx was currently away, healing from the injuries dealt to him by Oswald. He also recognizes Brom, explaining that he was the one who commissioned Brom's first Psypher. The pair attempt to escape, only for Valentine to appear in their way again and sick the last dragon Leventhan on them. Mercedes manages to defeat the fledgling world-killer, and the pair escape from the mad king's wrath.
Back at Ringford, Brom reforges Mercedes' Psypher Tasla, renaming is Riblam. He then informs Mercedes that it is the only weapon in the world able to destroy a Psypher. He then takes his leave, wanting to be with Oswald now that he's learned the Shadow Knight is alive, out of feelings of guilt over his involvement in Oswald's fate.
Mercedes rallies her forces for a last attempt to retake the Cauldron, and sends them to battle with a stirring speech that demonstrates how well Mercedes has inherited her mother's spirit. The Vanir manage to defeat the Aesir and send them into a retreat, and Mercedes meets Odin on the battlefield. Odin insults Mercedes and Elfaria, and claims that his victory has come willingly to him. The two do battle, and Mercedes proves she's not as helpless as Odin suspected, destroying Balor and wounding the demon lord. She then forces Odin to promise not to wage war on the fairies again and stay in his own kingdom, threatening him with execution and an invasion of Ragnanival if he disagreed. Odin agrees to the terms, and limps back to Ragnanival to lick his wounds with what few soldiers he had left. Meanwhile the Fairies cheer on their new queen, a time of prosperity on the horizo