"The Seven Deadly Sins" is set in the fictional kingdom of Britannia, a land where humans, giants, fairies, and demons coexist. The story revolves around the titular group of knights, the Seven Deadly Sins, who were once the most powerful and revered warriors in the kingdom. However, they were framed for the assassination of the Great Holy Knight, Zaratras, and subsequently disbanded. Ten years later, the kingdom is under the oppressive rule of the Holy Knights, who have overthrown the monarchy and plunged Britannia into chaos.

The narrative begins with Elizabeth Liones, the third princess of the kingdom, seeking out the Seven Deadly Sins to help her reclaim the throne and save her sister, Margaret, from the Holy Knights' tyranny. Her journey leads her to Meliodas, the leader of the Sins and the Dragon's Sin of Wrath, who runs a small tavern called the Boar Hat. Despite his youthful appearance, Meliodas is a centuries-old demon with immense strength and a mysterious past. Together, they embark on a quest to reunite the scattered Sins and confront the Holy Knights.

The other members of the Seven Deadly Sins include Diane, the Serpent's Sin of Envy, a gentle giantess with earth-manipulating abilities; Ban, the Fox's Sin of Greed, an immortal thief with regenerative powers; King, the Grizzly's Sin of Sloth, the fairy king who wields the sacred treasure Chastiefol; Gowther, the Goat's Sin of Lust, a doll-like being with the ability to manipulate memories; Merlin, the Boar's Sin of Gluttony, a powerful mage with vast knowledge and magical prowess; and Escanor, the Lion's Sin of Pride, whose strength fluctuates with the sun, making him nearly invincible during the day.

As the Sins regroup, they uncover a deeper conspiracy involving the demon clan, the Ten Commandments, who seek to resurrect their leader, the Demon King, and plunge the world into darkness. The Sins must confront their own pasts, including Meliodas's connection to the demon clan and his tragic relationship with Elizabeth, who is revealed to be the reincarnation of a goddess. The story explores themes of loyalty, redemption, and the struggle between good and evil, as the Sins fight to protect Britannia and its people from both human and supernatural threats.

The series is structured around multiple arcs, each focusing on the Sins' battles against various adversaries, including the Holy Knights, the Ten Commandments, and other powerful entities. The narrative builds toward a climactic confrontation with the Demon King and the resolution of the Sins' personal and collective struggles. Throughout the series, the characters' relationships and backstories are gradually revealed, adding depth to their motivations and actions. The story concludes with the Sins achieving their goals, but not without significant sacrifices and challenges along the way.