Having defended against the first wave of the Alvarez Empire's assault, the Fairy Tail members gather at the guild hall to discuss their dilemma of being surrounded on all sides. Though Mavis originally disagrees with getting others involved, she eventually accepts the idea of Mages outside of Fairy Tail helping. While Lucy tries to question the now-captured Brandish, the western army of the Alvarez Empire, led by Zeref himself, receives the news of Brandish and Ajeel's defeats. Upon hearing this, Zeref smiles, noting that otherwise, the things would be boring, however, he also points out that Fairy Tail not killing their enemies is their weakness. Meanwhile, the news from the east arrives and Fairy Tail Mages learn that Bosco has been destroyed and so have all their guilds. However, the Mages of Fiore also move out, with Sabertooth and Blue Pegasus going to intercept the army coming from the north, and Mermaid Heel and Lamia Scale heading out to liberate Hargeon Town in the south. Several Fairy Tail Mages are sent to help those groups out, however, they soon notice that Natsu, who is supposed to join the group heading north, is nowhere to be found. After it is revealed that Four Gods of Ishgar will face the greatest threat coming from the east, Natsu's location finally becomes known and everyone witnesses the Fire Dragon Slayer flying directly towards Zeref.
Natsu's act causes discord to erupt between Erza and Gray, with the two disagreeing on whether to trust Natsu and his reckless action; eventually, the Mages agree to follow the original plan and leave Zeref to Natsu. Meanwhile, Natsu makes the first contact with the enemy forces, charging straight through them with an intention to make Zeref appear. He takes out a huge number of soldiers, using his Magic for both offense and defense as the Alvarez soldiers fire back with their ranged weapons. Initially being overwhelmed by their relentless attacks, he quickly responds to their attacks in kind with Fire Dragon King's Roar, wiping out almost thousand soldiers in an instant. Ultimately, Zeref does appear, causing Natsu to point out that he thought it would take more than what he did to make the Black Wizard appear. As Zeref replies that he wanted to see Natsu as soon as possible, the Fire Dragon Slayer gets ready, removing the bandage from his right arm. Before they start, however, Zeref orders Invel to move the army away; Natsu tells Happy to fly away as well.
As Natsu states that they will not hand over Mavis and Zeref blames the current circumstances on Natsu, claiming that he was unable to destroy him, the Fire Dragon Slayer reveals the previously hidden tattoo of a Dragon on his right arm. Zeref notices an immense amount of Magic Power, as well as heat, coming from the Dragon Slayer; Natsu smirks as he charges at Zeref, delivering a devastating punch (enhanced by his flames) that sends Zeref crashing through a nearby hill. However, he jumps out and shoots his Black Magic towards Natsu, which the Fire Dragon Slayer incinerates, much to Zeref's shock. Making the use of his speed, Natsu instantly appears in front of Zeref and before he can react, the Black Wizard takes yet another attack from Natsu: an attack that keeps growing in power as an entire landscape falls to Natsu's flames.
The Dragon Slayer continues to barrage Zeref with a number of attacks, in spite of this, Zeref smiles and launches a counterattack that gashes Natsu's face. Undeterred, Natsu keeps clashing with his foe; Zeref backs away for the moment to comment on Natsu's newfound power, wondering where it came from. Natsu says that he "inherited" it from Igneel, explaining that it is the manifestation of the last remains of Igneel's Magic Power that was inside him, and that it took ten months of training for him to be able to properly use it. Zeref is surprised and Natsu further explains that after he uses the Magic Power, it will never regenerate. Zeref finds hope in these words, realizing that as it is a power granted from the dead, it may be able to destroy an immortal. Natsu then taps into this power and activates Fire Dragon King Mode; a surprised Zeref commends this, watching as even the ground begins burning. Natsu yells out that he is going to end the fight and Zeref, relieved that his wish is about to come true, gives his thanks to Igneel, Anna, Layla and Natsu, as his little brother hits him with the attack, seemingly destroying the Black Mage completely.
Invel watches all of this, surprised; Happy's smile soon changes to a worried expression as Natsu falls on his knees, exhausted. The Dragon Slayer claims that he's okay, saying that he has enough of power for one more strike, just as the dust clears out, revealing the figure of a now-heavily-injured Zeref. While Natsu begs Igneel to lend him his power for one more blow, Zeref smiles and states that the Dragon Slayer has attained great strength. The Black Wizard reveals that he has always hoped that Natsu would be able to stop him; however, he also says that he thought it was too late as Zeref had decided to destroy the world instead. Seeing Natsu's current strength, Zeref once again begins to hope that Natsu would be able to end him, so he decides to reveal something important: his name is Zeref Dragneel and his identity is that of Natsu's older brother. This greatly shocks Natsu as Zeref further explains that four hundred years ago, his parents, along with his little brother Natsu, were killed by Dragons and so Zeref began his research on life and death and wound up creating the Etherious, as well as reviving Natsu, who became E.N.D.: "Etherious Natsu Dragneel".
Natsu refuses to accept this, claiming that he's heard several times Igneel failed to defeat E.N.D. . Zeref states Igneel loved Natsu so much that he couldn't bring himself to harm to him. Natsu then mentions that E.N.D was the one who created Tartaros, but Zeref corrects him, mentioning that it was just Mard Geer using the Tome of E.N.D. to band the Demons together with a common goal. Summoning the very book, Zeref shoots through it, causing Natsu immense pain thus proving their link and proving his story; the Black Wizard goes on, explaining that he and Igneel were friends and that he eventually let Igneel take care of the now-revived Natsu and he, along with four other children and their Dragons, traveled through Eclipse to the present from four hundred years ago thanks to Anna and Layla's assistance. Zeref continues, claiming that waiting four hundred years was difficult, as a lot happened, until he mentions Mavis, which is when Natsu loses his patience and charges at his brother. In the middle of this, Zeref reveals one more thing: as a Demon from the Books of Zeref, Natsu will cease to exist if his creator dies. Natsu firmly states he doesn't care and declares his resolve of wanting to erase Zeref. However, Happy stops Natsu from delivering the crippling blow, despite Natsu's protests, as he mentions his reason for doing so being that he doesn't want Natsu to die. With Igneel's power fading, Happy yells out that Natsu is his best friend and that he is taking him home. Natsu reluctantly accepts Happy's resolve and the two fly away, while Zeref, with a smile on his face, thinks to himself that Natsu is no longer capable of stopping him now, and bids his brother farewell.
Afterwards, Invel approaches Zeref, witnessing the Emperor's injuries disappear instantly. Zeref states that as there is no one capable of stopping him anymore, and that any hesitation he had has dissipated. He tells Invel to continue the army's advance, intending to conquer Fairy Tail and capture Fairy Heart. As Invel accepts these orders, Zeref presents one more request: new clothes more befitting an Emperor for him. Meanwhile, Natsu and Happy stop by a lake inside a forest to rest. Natsu wonders if Happy's paws are alright and as the Exceed washes them in the water, he states that they are just fine. Recalling what just happened and what they learned about Natsu's identity, Happy speaks up, claiming that the two of them will always be best friends no matter what. As Natsu confirms it, Happy runs towards him, hugging him and, with tears in his eyes, promises to find another way to defeat Zeref without endangering Natsu's life.
Meanwhile, the battles against the Alvarez Empire proceed into the next stage: Lamia Scale and Mermaid Heel commence the mission to liberate Hargeon Town and stand against Dimaria and Wall, and their armies; to the north, Sabertooth and Blue Pegasus fight off the soldiers and meet up with the fearsome Bloodman; and in the east, the remaining Four Gods of Ishgar and Jura face the traitorous God Serena, as well as August and Jacob.