The battle against the forces of evil is nearing its end as the Undead King Leonis Death Magnus remains the sole survivor of the Dark Lords. Standing no chance alone against his enemies, Leonis seals himself deep underground with the goal of reincarnating a thousand years later and rebuilding his demonic army. Within the span of a millennium, however, everything has changed. People no longer remember the great war that led to Leonis' defeat and instead face a new threat—the monstrous creatures known as Voids. Magical devices have replaced sorcery, and humans with special powers—manifested in the form of weapons called Holy Swords—train at the Excalibur Academy to fight the Voids. Reborn as a 10-year-old human boy, Leonis meets Riselia Ray Crystalia, a dutiful girl who stumbles upon the ruins of his hibernation chamber. Believing that Leonis has amnesia, Riselia offers him guidance and enrolls him at the Excalibur Academy. Leonis still intends to take over the world, but he must first familiarize himself with a reality completely different from all he knows.

Romeo is a kindhearted and courageous boy living with his family in a small village in Switzerland. Unfortunately, Romeo becomes the interest of a man named Luini, known as The God of Death, who is infamous for buying children and selling them as chimney sweeps in Milan. While visiting the village, Luini burns down Romeo's family cornfield in an attempt to have Romeo as his own. With the cornfield gone and his father sustaining a head injury trying to put out the fire, Romeo bravely sells himself to the God of Death in order to help his family afford a doctor. On his way to Milan, Romeo meets a boy named Alfredo Martini and they quickly become friends. Just as Alfredo is sold to a different master, the two boys swear eternal friendship and vow to meet again. As a chimney sweep, Romeo faces many hardships and abuse, especially from his master's family and a gang known as the Wolf Pack. But after reuniting with Alfredo, the two form a fraternity of chimney sweeps called The Black Brothers, who will learn to fight against the Wolf Pack and help each other in times of need.

In the world of Loveless, each person is born with cat ears and a tail, which disappear only if that person engages in a sexual intercourse. Because of this, they have come to symbolize virginity and innocence. Additionally, fighting is only done by fighting pairs or couples, where one is known as the Sacrifice and the other as the Fighter. The first receives the damage while the latter attacks. Ritsuka Aoyagi is a 12-year-old boy, who for some unknown reason suffers from amnesia. His brother got killed recently, and as if his life has not been hard enough lately, on his first day at the new school he gets approached by a stranger called Agatsuma Soubi, who claims to have known his late brother. Ritsuka finds out that Agatsuma and his brother used to be a fighting pair, and that Agatsuma has inherited Ritsuka now that his brother is gone. Together, they try to find the truth behind his brother's death and the organization known as the Seven Moons, which may have been responsible for it. All the while, it seems that Ritsuka and Agatsuma are becoming closer than they intended to be…

In the American city of Snowfield, a new Holy Grail War is about to erupt. Such ruthless confrontations pit against each other magic experts known as magi, who ascend to the status of Master by summoning Servants—heroic spirits of the past. The winner is thought to have any one of their wishes granted. However, it appears that the organization behind the present situation has merely managed to create an imperfect copy of the Third Holy Grail War. As the unusual ritual catches the attention of the Mage's Association in London, Flatt Escardos, the young apprentice of Lord El-Melloi II—a former participant of the Fourth Holy Grail War—travels to Snowfield and, to the great dismay of his teacher, joins the ranks of the Masters. In this fake conflict where the line between Masters and Servants is blurred, the different factions will have a hard time preventing the city of Snowfield from plunging into total chaos.