Elizabeth Liones, the Third Princess of the Liones Kingdom, escapes from the traitorous Holy Knights to track down the notorious criminal organization, the "Seven Deadly Sins", in order to save the Kingdom. In her search she encounters a strange tavern, the Boar Hat. The owner is a small boy named Meliodas, who is accompanied by a talking pig. The Kingdom's knights soon catch scent of Elizabeth's plot and attempt to capture her. During the struggle she is saved by Meliodas, who reveals himself to be the captain of the Seven Deadly Sins. With one Sin found, the pair travel to Bernia to find information that could help locate the others, and their mission to reunite the Seven Deadly Sins begins.
Hearing from a customer in Bernia that the Holy Knights go out of their way to avoid the Forest of White Dreams, Meliodas deduces that a Sin must be hiding out in the forest. In the forest they encounter a sleeping giant, revealed to be the Serpent's Sin of Envy, Diane. Having reunited with Diane, the group encounters the Holy Knight, Gilthunder. Meliodas engages Gilthunder in combat and obtains from him the locations of two of the Sins, but is wounded during the battle.
Using the information they obtained from Gilthunder, the group heads to Baste Dungeon where Ban, the Fox's Sin of Greed, is being held prisoner by a group of Holy Knights called the "Weird Fangs". While travelling, Meliodas succumbs to his injuries, forcing the group to stop in Dalmary and wait for his recovery. The Weird Fangs use this time to strike at the Sins and Diane makes her way to Baste Dungeon to combat the threat. When Ban hears of Meliodas' return, he effortlessly manages his own escape and the Sins engage in a battle with the Holy Knights. Meliodas' extraordinarily fast recovery reveals a dangerous side to the Dragon's Sin of Wrath.
Having found three Sins, the group seek out King, The Grizzly's Sin of Sloth, who is said to be resting in the Capital of the Dead. The group soon encounter him, but the reunion doesn't go as planned. King is working with the Holy Knights in order to kill Ban, whom be blames for his sister's death. King and the others eventually make their way into the Capital of the Dead, where Ban is reunited with King's sister, Elaine. The Holy Knight, Guila, uses this opportunity to engage the Sins in combat, and they are quickly overwhelmed by her strength. While the Sins defend against Guila, King learns that Ban's connection to Elaine is more profound than he understands.
While the Holy Knights gather in Liones to prepare for the prophesied Holy War, the Sins seek information on the whereabouts of their remaining allies and the locations of their sacred treasures. In their search they encounter one of Elizabeth's sisters, who, with the help of her bodyguard, attempt to kidnap Elizabeth. Swiftly rescued by Meliodas, they escape to Vaizel, where the Sins regroup following information that could lead them to a sacred treasure. While there, they participate in an annual fighting festival and discover the prize is Diane's "Gideon". As the festival reaches its conclusion, Meliodas is reunited with an old comrade, but before the final match can begin, the festival is halted by the arrival of three Holy Knights who launch an assault on the town. As the Sins fight off the "Reactors"; King is unsettled after Guila's master reveals his identity, and the mystery behind Meliodas deepens. In the capital, the mastermind behind the attack schemes a sinister plot behind the back of his counterpart.
Moving past the destruction of Vaizel and the loss of Meliodas' sword, the Sins find themselves following a rumor to the tranquil village of Ordan. Meanwhile, in the capital, the Holy Knights report that a mysterious "Armor Giant" has been located in the mountains surrounding Ordan. Helbram orders the infamous Holy Knight group "Dawn Roar" to dispose of the giant, and they leave immediately to carry out their mission. The two groups soon clash and engage in battle over the giant they believe to be the Goat's Sin of Lust, Gowther. However, when the real Gowther reveals himself amid the fighting, the Sins learn that the Holy Knights may be the least of their worries.
With the fifth Sin found, Meliodas rallies the group together in order to retrieve his stolen sword. Though initially weary, the Sins are forced to act after Elizabeth is captured by an enigmatic, masked woman in connection with the Holy Knights. Spirited away and left to brood in a dungeon cell, Elizabeth is reunited with her elder sister. The Sins, wasting no time, infiltrate the kingdom and begin a number of skirmishes with the Holy Knights. Meanwhile, a king from the south and his mysterious robed master ask for an audience with the king. The fairy king bids a final farewell to his estranged friend as the two take to the skies and settle a centuries old conflict. Ban searches for the Horn of Cernunnos and makes contact with the Goddesses, but discovers their bargain comes at great cost. As the fighting rages on, Hendrickson makes his final move to open the Coffin of Eternal Darkness and resurrect the demon clan.
Peace returns to Liones with the defeat of Hendrickson, but the threat of holy war still lingers over Britannia. As the people celebrate their victory and rebuild their capitol, Ban departs from the Sins. Accompanied by King and Jericho, he journeys to the Fairy King's Forest in order to find a way to resurrect his lost love, and King's sister, Elaine. Back in Liones, King Bartra warns the Sins that he has seen a vision of great calamity to the south, in the fledgling kingdom of Camelot.