Arrests at Mercurius is an event that takes place in the Grand Magic Games arc.
After their talk with the summoned soul of the Dragon Zirconis is cut short, the Fairy Tail Mages in the Dragon graveyard find themselves confronted by Arcadios and Yukino Agria. The Mages are surprised to see the two of them, who tell them that since Zeref is the root of evil, defeating him would be the key to defeating Acnologia. Arcadios introduces himself as a member of the Fiore Army while Yukino does the same in her position as the temporary sergeant.
When the Mages begin wondering why Yukino is there, Arcadios explains that a strategy they're working on, which requires the power of a Celestial Spirit Mage, is in progress. As Natsu asks what he's talking about, Arcadios begins complimenting him on his performance in the games, at which point Natsu angrily gets in his face, asking why he needs a Celestial Spirit Mage. As Happy tells Natsu that Arcadios has a high rank in the army, the latter walks away and tells the others to follow him. Yukino asks Lucy to follow as well, stating that with their plan, they can defeat Zeref and Acnologia.
Arcadios and Yukino bring the Mages to Mercurius castle and take them inside. Arcadios apologizes to them for his attempt to kidnap Lucy days before, stating that he needed a Celestial Spirit Mage and was prepared to go to any lengths. Arcadios explains that the Grand Magic Games had been started to gather the Magic of participating Mages so it could be used for their project: Eclipse. Arcadios shows the Mages a large machine to be used in the project, stating at the solar eclipse, one will be able to travel back in time and defeat Zeref before he was immortal. Yukino explains that because time travels differently in the Celestial Spirit World, they needed a Celestial Spirit Mage to help them cross over. Arcadios states that the project will go through on July 7, which the Dragon Slayers note to be the day their Dragons disappear and wonder if it's just a coincidence.
Suddenly, the Mages, Yukino and Arcadios are surrounded by members of the Fiore Army, who make way as Darton, the Minister of Defense, approaches the group. Arcadios asks the meaning of his actions, to which Darton asks him for the reasons behind his own, telling him he shouldn't reveal information of national importance to outsiders. Arcadios defends himself, saying that they're key to the plan, which Darton is against. Darton responds that he is against it due to the potential dangers of changing history, which makes the Mages reflect on its implications. With that, Darton orders the arrest of Arcadios, Lucy and Yukino. Natsu attempts to stop him but has his Magic removed by the Eclipse machine due to its proximity. Unable to help, Lucy and Yukino are arrested.
The rest of the Fairy Tail Mages are ejected from the castle by Darton afterwards. Darton tells them that Lucy will remain where she is. However, due to the King's love of Fairy Tail, if they win the Grand Magic Games, they will be allowed an audience with him, which they accept as currently their only way to save her.
The Mages return to their lodgings to discuss what to do about Lucy's arrest. Natsu, restrained to a pillar, states they should go rescue her right away, but Wendy and Happy try to convince him otherwise. As everyone wonders what to do, Makarov states that they can't act rashly against the army but they can't allow Lucy to remain captive. Natsu breaks free once again, demanding to go save her but is pounded down by Makarov, who tells everyone that though they won't simply charge into battle, they won't avoid it either.