News about several sightings of an Armor Giant somewhere in Ordan Forest reaches Helbram, who quickly sends Dawn Roar, an elite group of Holy Knights, to search for the Armor Giant. Meanwhile, Guila visits her younger brother Zeal, and she later kills three thugs who bullied him for his spare change. Meliodas and his friends split up in the nearby village. Elizabeth and Hawk encounter a boy named Alan, who is looking for some glue to repair armor. Upon hearing the low growls that echo in the forest, Meliodas, Ban and King find the Armor Giant being surrounded by Dawn Roar, but Meliodas, Ban and King are no match against Dawn Roar. Just as it seems that the Armor Giant is about to be slain, Alan intercepts and reveals himself as Gowther, the Goat's Sin of Lust.

Gowther gives the head of the Armor Giant to the members of Dawn Roar in order to convince them to withdraw from battle. However, the demon breaks through the armor and attacks Meliodas, Ban, King and Gowther. Meliodas lowers his guard upon noticing a Holy Knight merged into the demon's chest, but Gowther uses his Sacred Treasure, the Twin Bow Herritt, to stop the demon. Meanwhile, Cain visits Elizabeth in the Boar Hat, where he tells her the story of Liz, a knight from an enemy kingdom who became Meliodas's past lover after he saved her from being sentenced to death. Ban gouges out the heart of the Holy Knight, recognized as Guila's father Dale, yet the demon still remains alive. Elizabeth rushes to Meliodas and gives him Liz's sword left behind, which allows him to destroy the demon. It is realized that Dale was one of Hendrickson's failed demon experiments.

Gowther is welcomed by the others into their group, but he soon finds out the true secrets behind them by delving deep into their memories and emotions. Meanwhile, Hendrickson and Helbram have the hilt of Meliodas's sword in possession, in which they plan to use it as a fragment of a ritual relic, known as the Coffin of Eternal Darkness, to resurrect the red demon race as a means to start a holy war. At the Boar Hat, Holy Knight Vivian suddenly appears and kidnaps Elizabeth and Hawk. Diane throws Meliodas, Ban and Gowther toward the Kingdom of Liones, where the three are bombarded by an army of Holy Knights on the north side of the castle. However, Arthur Pendragon, the young king of Camelot, has approached the south side of the castle with his own army.

Hendrickson tries to convince Arthur to leave the castle, but to no avail. Instead, Hendrickson escorts Arthur inside the castle and suddenly engages him in battle. Meanwhile, Elizabeth and Hawk are teleported inside the dungeon of the castle, and they try to find a way out. Meliodas, Ban and Gowther manage to defeat the army and sneak inside a town of the kingdom, while King summons his black hound Oslow to use its mouth to teleport Diane inside the castle. However, Diane is attacked by Guila, Jericho, Gilthunder, Howzer, Helbram and Great Holy Knight Dreyfus. Diane tries to defend herself, but Dreyfus overpowers her with an attack that sends her crashing onto the town streets. After Diane wander the streets and protects Zeal from being crushed by a falling building, both Howzer and Guila switch sides to defend Diane.

The combined powers of Howzer and Guila have little to no effect. When Dreyfus attacks Guila, Gowther saves Guila in the nick of time and faces Dreyfus alone. Meanwhile, Elizabeth and Hawk encounter Margaret Liones, Elizabeth's eldest sister, in a cell. Vivian appears, teleporting Hawk elsewhere, knocking Elizabeth unconscious and releasing Margaret from her cell. Gowther traps Dreyfus in a nightmare, but Dreyfus somehow snaps out of it and pierces Gowther in the chest. Once a heavily injured Dreyfus leaves with Gilthunder, Helbram attempts to finish Diane off, but King appears and stops Helbram, who reveals himself as a fairy and King's former best friend. Helbram uses his magical tree roots to restrain King and inflict brutal lashes on Diane. However, King uses Chastifol to shield Diane from getting hurt. King says that he is fulfilling a promise that he made seven hundred years ago.

Gilthunder leaves Dreyfus to fight Meliodas alone, while Hendrickson deals with Arthur and ultimately defeats him. Meanwhile, Elizabeth wakes up in a chamber and finds her father, King Bartra Liones. Meliodas then finds himself outmatched against Gilthunder, Hendrickson and Vivian. At the same time, Margaret decides to take her own life by jumping off a castle tower. Margaret and Gilthunder are both released from their curse in the form of monsters, thanks to Meliodas. While Margaret is thus saved from falling, Hendrickson tries to finish Meliodas off, but Gilthunder stands in the way and knocks down Hendrickson at full force. Vivian tries to trap Meliodas, Gilthunder, Margaret and Arthur in illusionary places, but Merlin, the Boar's Sin of Gluttony, makes her first appearance and breaks the spell.

Ban explores the catacombs under the city. Merlin and the other sins go to the chamber where the king Baltra and Elizabeth are being held captive. Dreyfus has a confession.

Ban attempts to kill Meliodas, but is stopped by Hawk. Meliodas and the sins corner Hendriksen and prepare to take him down.

Hendrickson unveils the source of his power, the corpse of a red demon, which is the same one that Ban slayed in the Fairy King's Forest long ago. After Ban pummels Hendrickson through the ground, Meliodas, Ban, King and Gowther jump down the hole to a cave, where Hendrickson injects himself with the blood from the corpse of a gray demon to acquire even more power. A sudden burst of energy from Hendrickson sends the four of them back above ground, where Hendrickson easily overpowers all of his opponents despite all of their efforts. With Meliodas left standing and protecting Elizabeth, Hendrickson casts a large dark blast, but Hawk sacrifices himself in order to save the two of them. Shocked of losing her dear friend, Elizabeth awakens her dormant powers that repels Hendrickson and heals everyone else. However, Hendrickson survives and still has strength left in him to fight.

The Seven Deadly Sins and the Holy Knights all combine forces in an attempt to defeat Hendrickson. Meliodas, given support from his friends and allies, uses his ultimate technique to finish Hendrickson off. When Bartra appears with Merlin, he commands the Holy Knights to restore the kingdom and preserve its people as their "punishment" for starting a holy war. Bartra then thanks the Seven Deadly Sins for saving the kingdom and protecting Elizabeth. Hawk is revived as a piglet, much to a surprise. Things return to normal inside the kingdom, and a festival is held at night. The next day, Elizabeth joins Meliodas, Hawk, Diane, Ban, King, Gowther and Merlin in finding the last member of the Seven Deadly Sins, Escanor, the Lion's Sin of Pride.

Ban and Meliodas meet to continue the duel they left unfinished before, only to find that Hawk has told everyone about their solemn battle.

Business at the Boar Hat is dropping, so Meliodas asks Merlin to join the staff as a waitress alongside Elizabeth and Diane to attract customers.

As the kingdom begins to rebuild after the coup attempt, the king honors the Seven Deadly Sins for their bravery with a medal ceremony.

With Fraudrin's manipulation of Hendrickson, the Ten Commandments return to the physical realm. King follows Ban to the Fairy King's Forest.

Diane confronts Gowther after he manipulates Guila and Zeal's memories. With the return of the Demon Clan, new threats descend upon allied clans.

The fairies reconcile their differences with King as Ban leaves the forest. Meanwhile, Meliodas and gang are overwhelmed by the sheer power of Galand.

Hendrickson, thought to have been defeated against Meliodas, aids Gilthunder and the Holy Knights against a powerful Gray Demon.