For 15 years, a war has raged between the humans and demons. The human's greatest warrior, Hero, heads to the castle of the Demon King, leader of the demons to end the war. To his surprise, the "Demon King" is in fact a woman and she has been waiting to meet Hero in person and wishes to talk with him instead. As they talk, she reveals the war has actually benefited both sides as their races have put aside their differences to fight against a common enemy and improved the economy. However, it has allowed the powerful and corrupt to abuse the weak particularly how the poorer Human Southern kingdoms must continue fighting the demons in exchange for food and financial aid from the rich Central kingdoms. Furthermore, no matter which side wins or if the war suddenly ends without compensating those employed because of the war, it will lead to a massive civil war much to Hero's horror. But the Demon King has a plan to bring prosperity and a lasting peace for both their races and wants his help. Convinced, Hero agrees where both of them pledge their lives to each other.

The Demon King’s plan is met with heavy resistance from uneducated humans. The Hero gets to witness and the darker side of the Head Maid and questions the validity of the grand plan.

With the first harvests coming in and things settled in the village, the Demon King visits a convent for the next step in her plan. But the nun she meets there has a past with the Hero, and isn't happy to see them together at all.

Several months had passed since Hero's departure and Female Knight moved to the villa to teach swordsmanship to the Crimson Scholar's students. A member of the Trade Alliance, Young Merchant, pays a visit to Crimson Scholar and she introduces to him another crop then unknown in the Human Realm, corn, claiming that it will help boost mankind's food production even further. Before leaving, he proposes to Crimson Scholar much to her embarassment. With half a year since Hero left to the Demon Realm, the Queen's anxiety increases, unaware that he has made use of his teleportation skill to return regularly to the villa behind her back, a fact only known by Chief Maid, to whom he confesses that he refrains to meet her out of his drive to become useful to her. Meanwhile, the southern nations are assembling a huge naval force to recapture Aurora Island, the only territory in the human world in possession of the Demon Army.

At Gateway City, a huge city in the Demon Realm occupied by the Humans, Hero, addressing himself as Black Knight, saves a Demon barmaid from being harassed from Crusaders and learns from her how most of the Human army have mistreated the Demons since occupying the city except those who guard the Eastern Fortress. Meanwhile, the expeditionary fleet sent to recapture Aurora Island is a disaster no thanks to their incompetent commander the White Night King when Demon Krakens destroy most of the fleet with only a few survivors, including the Winter Prince, whose father is killed saving the White Night King. Back in the Southern Nations, most of the Southern leaders are disgusted with White Night King, who refuses to take responsibility for his failures and tries to blame it on the late Winter King. Viewed with distrust by White Night King due to Crimson Scholar, the Church and the Alliance helping the Winter Kingdom, the Winter Prince assumes his father's throne, becoming the new Winter King and summons Female Knight to command a second expedition to recapture Aurora Island. As Female Knight prepares to depart, Scholar confesses her true identity to her and wants to be her friend. To her surprise, Female Knight already knew since Hero told her the truth before and accepts the Queen's friendship. The New Year Festival arrives and after one year apart, Hero and Demon Queen finally reunite. After the two have a dance by themselves, Hero reveals to her that he has a plan to liberate Gateway City in one month and learns from her that she also intends to help the Humans retake Aurora Island. Hero then leaves again, promising to meet her next time in the battlefield.

The Crimson Scholar offers plans to the Winter Kingdom to capture Aurora Island. Hero liberates Gateway City by terrifying the nobles to an extent which makes them flee the city. The commander flees after selecting the new commander of the city, of which this new commander decides to make Gateway City a new government that allows humans and demons to live together. The Crimson Scholar unfolds the plan to create an artificial land bridge linking the Winter Kingdom and Aurora Island. After capturing the beachhead, they learn that the Ice General is currently awaiting to be besieged, with an army of 8,000. However, the Winter Kingdom only has an army of 12,000; when normally three times the amount, 24,000, is necessary to successfully siege a castle. However, Hero arrives with a grand army of 9,000 of the best soldiers the Humans have. These soldiers are actually those who had fled the Gateway City, led by the previous commander of Gateway City to aid the Winter Kingdom. Due to this, the Ice General immediately launches his entire army to attack Winter Kingdom, but is soon defeated. The Winter Kingdom has now captured Aurora Island. While the kingdom is celebrating, Hero is welcomed back to the Winter Kingdom by Rou Yumihei, whom he refers to as old man.

As the second summer arrives, the Winter Kingdom has improved significantly with the Crimson Scholar sending one of her students, the Merchant Youngster to help the Winter King. After training with Female Knight and tasting Younger Sister Maid's carbonated drink, the Hero heads back to Gateway City as the Black Knight to attend a festival. While feasting with the Eastern Fortress General, the Fire Dragon Princess, daughter of the Archduke Fire Dragon, arrives much to Hero's chargin as the Princess considers herself as the Black Knight's wife. Realizing time is running out for her, the Scholar brings Hero along with Elder Sister Maid, whom has become her apprentice, to the Iron Nation to show a new invention that will improve the education of the humans, the Printing press. During one night when both the Queen and Female Knight sleep together with Hero, the Queen reveals to Female Knight that she will be going back to the Demon Realm to maintain her rule and go on a ritual that will renew her right to rule and ask her to protect Hero. Before she and Chief Maid head back to the Demon Realm, the Queen gives Elder Sister Maid a magical ring that will change her appearance to look like her so she can attend meetings in her place. Meanwhile, the Crusader Commander is sentence to death for cowardice and abandoning Gateway City but is rescued by the White Kingdom. Bitter over the Winter Kingdom's success, the White King and the Commander joins forces to devise a plot against them.

Young Merchant pays a visit to the Crimson Scholar's villa and by her instructions, Elder Sister Maid tries to impersonate her with the magical ring, but is unable to deceive him. Hero then intervenes and takes him to the Demon realm where he shows him that the now liberated Gateway City became a thriving metropolis inhabited and ruled by humans and demons alike and have the Fire Dragon Princess hold a banquet to them, where he offers him the right to explore the commerce in the city in exchange to help in his and Demon King's efforts. Meanwhile, the Queen has a short meeting with Female Magician at the Royal Library, before entering into the ritual chambers, instructing Head Maid to kill her should she loses her sanity in the process. Back at the villa, Female Knight offers herself to be properly knighted by Hero in a sign of offering him her eternal loyalty, to which he reluctantly accepts. The Winter King is visited by a representative of the church who orders him to hand over Crimson Scholar, who was branded as a heretic and her teachings blasphemous by the newly elected Pope himself, and he rushes to the villa with Archer to warn the others. To protect the Winter Kingdom from further problems, Hero comes up with a plan, by having Elder Sister Maid, disguised as the Scholar, handed over to the church just to be rescued by Hero in a later occasion.

Young Merchant learns from his sources that the Central church and Central nations concocted the heresy charges against Crimson Scholar as her work was helping the South being free from Central's influence. Taking action, he has one of Scholar's students, the Noble Youngster, help in his plans. Meanwhile, an Emissary of the church arrives to take Scholar into custody, unaware that she is in fact Elder Sister Maid in disguise. Before a huge crowd, she is accused of heresy, chained and tortured, as Hero awaits for the opportunity to rescue her, but just when Elder Sister Maid is given the chance to speak, she tells everyone she was once a serf. She tells her story of how she lost most of her family during her days of serfdom and until thanks to their deity, the Spirit of Light, she was saved upon offered a chance to rise and stand as a proper human. She tells the crowd the Spirit gave humans the freedom to choose their own fate and no one has the right take it away from them, and allowing anyone to do that makes them insects. Angered, the Emissary orders the crowd to stone her but the crowd, inspired by her words, stone him and his entourage. One of Emissary's guards attempts to execute Elder Sister Maid but is saved by the Winter King, who puts the Scholar under his country's protection and orders the Emissary to leave with Female Knight also calling him and the Central church corrupt and declares Scholar a Saint. The Emissary flees and vows the Winter Kingdom will pay for their heresy. Meanwhile, a blue skinned demon has visions of Elder Sister Maid which disturbs him while a mysterious entity laments on what is happening right now.

The Southern kingdoms join forces to defy the Central nations by ending serfdom and lower taxes to encourage people to immigrate to the South and the Lakeside Convent separates from the Central Church to form the True Church of the Light Spirit with pamphlets and bards spreading their message. Meanwhile, Young Merchant begins buying wheat from the Central nobles with futures contracts which causes food prices to rise as part of his plans. Fearing their food supplies will run out if Central continues importing their food, the South enacts trade tariffs and taxes on exports. Meanwhile in Gateway city, Fire Dragon Princess informs East Fortress General that the Blue Demon clan are up to something while she later meets with Young Merchant in the Human Realm to buy salt as the demons are running out of salt. As Central declares war on the South due to their trade restrictions, Hero asks the Winter King to buy time before the battle begins as he tries to find a way to prevent needless bloodshed between both sides.

As Young Merchant and Fire Dragon Princess learns that Central intends to install a new currency to prevent inflation, Winter King and Hero are informed an army lead by the Blue Demon clan has emerged from the portal to the Demon Realm and intends to attack from the south. Hero departs to confront the Blue Demon clan alone, allowing the Southern Alliance to focus on the incoming invasion from the north and much to his surprise, he reunites with Female Magician on the way there. Meanwhile in the Demon Realm, the Queen emerges from the chamber possessed by the spirits of the ancient Demon Kings and Chief Maid manages to push her back inside, despite having her right arm cut off by her. Young Merchant, accompanied by Fire Dragon Princess exposes to Merchant Youngster his intention to have the Southern Nations establish their own currency and extend the Merchant Alliance trade with the Demon Realm. In the occasion, Fire Dragon Princess reveals her true self to him, and that she is an envoy of the Demons of Gateway City. The White Night Cavalry led by the Crusader Commander launches a surprise attack to the East of the Iron Kingdom in retaliation for their serfs leaving the country for the Winter, Ice and Iron nations. However, the attack fails thanks the advanced tactics Warrior Youngster learned from Crimson Scholar and Female Knight. Upon learning of the situation, Female Magician instructs Hero to destroy the magic portal linking both realms after she drives away the entire Blue Demon army by having all of them teleported away at once. Passing through the hole created by the explosion, Hero arrives at the Demon Realm and realizes that both Realms are actually located in the same world, like two sides of a coin. As Female Knight moves on with her plan to have the local peasants offer spoiled fodder to sicken the Central Army's horses and hinder their plans, Hero finally reaches the Queen, just to be attacked by her possessed self.

Hero and the Queen are finally reunited after she manages to keep the ancient spirits at bay and regain control of her body. Female Magician appears before the Winter King and delivers him instructions from Crimson Scholar to develop a vaccine for Smallpox and reveals that reveals herself as a demon. Crimson Commander invades the press building and attacks kill Elder Sister Maid, believing that she is Crimson Scholar, but she is rescued by Warrior Youngster who confronts the invader and have him fall to his death. After restoring Chief Maid's arm, Hero explains the situation to the Queen. Meanwhile at the surface, with their horses sick, the Central Army is stationed with their leaders arguing about the yet unclaimed spoils of the war, until a mercenary force makes the first move. Female Knight leads the battle to quickly defeat the mercenaries, and with the arrival of winter, the Central Army is forced to retreat. Back at the Central Nations, it is revealed that the Azure Demons, together with the Central Kings and the Church and are the true conspirators responsible for the war, prepare for resuming the conflict, now armed with firearms, whose design they stole from one of Crimson Scholar's collaborators in the Iron Nation. Back at the Demon World, Demon Queen and Black Knight appear before the Demons and the Queen summons a meeting with the other Demon leaders, which they see as a sign that the war with the humans will continue, much to her sadness. While Young Merchant discuss with the Southern Kings about opening trade with the Demons, Hero and the Queen return to their villa and reunite with their friends.