After the unsuccessful attempt to rescue Lucy Heartfilia and Yukino Agria from the prison within Mercurius, the Fairy Tail Rescue Team, along with the two Celestial Spirit Mages, falls deep into Mercurius' underground cavern, mislead there by Hisui E. Fiore, the King's daughter. They soon look for a way out, but encounter a heavily wounded Arcadios on the ground, advising them to run. However, before they are able to do so they are ambushed by a faint silhouette towering them all. Soon, the figure of the foe is revealed to be that of a hefty man possessing a beer in his hand. The Fairy Tail rescue team and the Celestial Mages manage to avoid his Acid Magic spell, but are soon faced with another enemy holding a flag, repeatedly shouting out "big catch," and making the terrain around said Mages' standing position rise. After being blown back by the pressure of their attacks, the Fairy Tail Mages and Yukino question their actions. Flowers begin to spring up from the cracks in the ground, encompassing a female figure within. Afterwards, an abundant amount of paper begins to gather at one spot, and form a female body when complete.
Arcadios, trying his best to mumble the desired words from his lips, warns the Fairy Tail Mages as well as Yukino. Telling them that they are face-to-face with an independent unit serving the Kingdom of Fiore from the shadows, the Garou Knights. Lucy, Yukino and the others prepare themselves for battle. The Garou Knights overwhelm the Fairy Tail rescue team with their outstanding coordination as well as the power of their attacks. After multiple attacks and assaults by the opponents, namely Kamika, Cosmos and a member of the Garou Knights, on Natsu Dragneel, Wendy Marvell, Mirajane Strauss and Panther Lily, Cosmos focuses a series of Vines towards the harmless Celestial Spirit Mages, but all in vain as the vines are intercepted by Lily in his battle form with his Musica Sword. As the battle rages on, Kamika uses her Paper Blizzard: Violet Dance, a paper to be said to be the god of blinding; immobilizing all of the Fairy Tail Mages as well as Yukino. Seconds after, Cosmos summons a large flower on top of the Mages.
The flower begins to suck them in, with Lucy being protected by Natsu and Yukino trying to hold Mirajane's hand whilst hovering above the ground. Wendy, however, uses a spell that returns mobility to the Mages previously affected by Kamika's spell. Natsu, Panther Lily and Mirajane decide to destroy the flower created by Cosmos. By combining their attacks and concentrating it to the flower, they manage to stop the absorption but create a strong explosion, scattering the Mages across different locations. Natsu is stationed with Kama, while Wendy faces off against Cosmos, at the same time Mirajane is paired with Kamika, and Lily with the Acid Magic Mage.
Moments after being scattered, Lucy, Yukino, Happy, Carla and Arcadios all end up in the same place. Happy complains about the pain felt by the rubble he is being squished by, while Carla notes the fact that all of the useless ones ended up in the same place. Happy picks himself up and, with his hand in the air, states that all they can do now is look for the others. Unexpectedly, Happy's body begins to float in the air, worrying the Celestial Mages. A member of the knights sits on a statue as he says that he caught a big fish, referring to Happy, causing Happy to angrily ask him if he looks like a fish.
However, before he can reply, he swings his hand, slamming Happy into a nearby pillar. Carla looks at the knight and ponders over his appearance. The knight introduces himself as Uosuke to the Mages standing before him, with Lucy admitting that his name is pretty. Happy gets up and states that they cannot lose to Magic like Uosuke's, with Carla in agreement with him. Uosuke, with an unchanging expression, tells them that if they take that tone with him, it will make him angry. Lucy and Yukino partner up and state that they can beat Uosuke, calling him "small fry." Arcadios, laying on the ground, warns them not to underestimate him, as legend says that all of his victims are reduce to nothing but bones, frighting both Lucy and Yukino.
Uosuke crosses his arms diagonally as he begins to cast a Terrain Effect Magic spell, altering the terrain around Lucy and Yukino. The ground begins to crumble, and rocks begin to fall into the burning lava beneath, with Lucy and Yukino holding onto the edge of the now deformed terrain. Arcadios desperately calls out their names. Happy and Carla fly to their rescue but are stopped half way by Uosuke's Terrain Effect: Gravity Zone, forcing their bodies to the ground as well as making them heavier. While Lucy and Yukino hang from the edges of the destroyed ground, Arcadios scans their current situation and notes that they are having a difficult time, and tells them to keep it up; since they are the only hope to achieving the Eclipse project, as he gets up.
Arcadios voluntarily walks on the lava as he states that he regrets nothing, even if he would have to forfeit his life for the Eclipse. Lucy and Yukino see this, and stare at him in shock. Uosuke questions if humans can move in lava, as he looks at the struggling Arcadios. Arcadios, finally reaching the Celestial Spirit Mages' location, pulls them up to the ground, telling them to hurry. Lucy and Yukino attempt to grab his hands to pull him up, but instead Arcadios tells them that once they manage to defeat the Garou Knights and escape from Abyss Palace to meet with Hisui E. Fiore, the princess. Before descending completely into the lava, Arcadios tells them that it's their choice to determine if Eclipse is the right path or not.
Yukino and Lucy look on as Arcadios submerges into the lava, crying out his name with a bitter expression. Uosuke cleans the sweat from his face as he is relieved to see Arcadios die. Seconds after, Horologium forcefully summons himself through the use of his automatic response mode when he senses that Arcadios is in grave danger. He rises from the lava with Arcadios inside his glass case, stating that he was able to save him in the nick of time. However, due to the pool of lava in which he is confined in, his body is unable to withstand the intense heat, as well as Arcadios' physique proving to be of great inconvenience.
Loke steps behind Lucy and Yukino, reminding Lucy that he is able to freely pass through the Celestial Spirit World's gate, and informs her that her 'stars' are with her, as he holds Lucy's keys in his right hand and hands them to her. Lucy is surprised to see him, and Yukino looks at him as she notes he is the Leo constellation. Loke looks back and hands her her keys, as Yukino blushes. Yukino holds her two keys, apologizing to them. With all twelve keys gathered, Lucy, Yukino and Loke get ready to counterattack, with Uosuke sweating at an extremely quick pace.
Happy informs Lucy that Arcadios is still breathing, signalizing that he is still alive, whilst Carla is shocked to find out he survived the inferno he experienced moments ago. Yukino, holding Pisces' key in her hand, clutches the key and begins to summon the latter, chanting "En Grande" as she summons it. The twin fish then begins to materialize, prompting both Happy and Uosuke to shout out the Celestial Spirit's appearance; fish, as well as Lucy questioning the motives for the Celestial Spirit being summoned.
As the spirit begins to take shape, Yukino explains that this is Pisces' true form, an entwined Celestial Spirit of mother and child. Once the smoke clears, a female body with various features of its celestial star, and a boy possessing a muscular structure appear. The boy states that if they were summoned in said form it means that they must terminate the enemy, with the mother agreeing with him. Standing next to Lucy, Loke notes that even though she has a kid, her beauty can omit such a thought from one's mind.
Pisces then charges at the enemy, with the boy calling Yukino his mother, but Yukino quickly tells him that she is not his mother. Uosuke rapidly takes action and uses his Terrain Effect: Gravity Zone, but Yukino counters by summoning Libra at the same time, telling her to use her Gravity Change to negate Uosuke's Terrain Effect: Gravity Zone. Uosuke is unable to use another one of his spell as Pisces bashes him with their Water Magic spells. Out of frustration, Uosuke uses Terrain Effect: Whirlpool Zone, alerting Pisces to change back to its fish form.
Yukino finds it surprising that Uosuke had known that Pisces' Achilles heel was water, much to Lucy's surprise. Lucy looks at her surroundings and states that, while holding her keys, if there is water she can summon Aquarius, the water bearer. Moments later, Lucy grabs Aquarius' key and summons said spirit to her aid. Aquarius sends a torrent of water towards Uosuke, knocking him unconscious and sending him spinning towards a wall.
The wall collapses due to the power and pressure of Aquarius' attack. On the other side of the now destroyed wall, Lucy and Yukino, as well as Happy, Carla, Loke and Arcadios, find themselves face to face with Natsu, Mirajane, Panther Lily and Wendy. They look at the five defeated Garou Knights laying on the floor as they congratulate one another on their victories. Natsu punches his open right hand with his left fist as he orders the Garou Knights to show them the exit, or he will show them his vanishing act. Lucy looks at Natsu and, after hearing Natsu's previous command, calls his intentions evil.
As Lucy and Yukino walk with the others, Yukino looks at Arcadios and asks Loke if he is okay, to which he replies by telling her that he's doing fine. On the other hand, Carla is still amazed that he was able to survive the lava pit. However, Yukino speculates that it must've been thanks to the amulet he has in his possession; a strong protection charm, that he made it out alive. As they keep looking for a way out, they come across a door that opens itself up. After fully opening itself, a hooded female figure appears in the entrance, shocking the others.