The "Seven Deadly Sins," a group of evil knights who conspired to overthrow the kingdom of Britannia, were said to have been eradicated by the Holy Knights, although some claim that they still live. Ten years later, the Holy Knights have staged a coup d'état and assassinated the king, becoming the new, tyrannical rulers of the kingdom. Elizabeth, the king's third daughter, sets out on a journey to find the "Seven Deadly Sins," and to enlist their help in taking back the kingdom.
After the defeat of demonized Hendrickson, The Seven Deadly Sins are commended for their bravery and courage by King Bartra. However, just before the ceremony, Ban and King leave. Earlier, Ban had told Meliodas that he is quitting the Seven Deadly Sins. As Ban leaves the Boar Hat Bar, King sees him and asks where he is going. Ban tells King that he wants to go to the Fairy King Forest. King then accompanies Ban on his journey. Meanwhile, it is reveals that Hendrickson has survived the assault from the Sins and he is shocked upon seeing Dreyfus alive. Dreyfus then summons the Commandments, ten demonized warriors who intend to destroy the Sins.
The Ten Commandments are in the process of recovering their magical powers and fly to the Kingdom of Edinburgh in the east. Meanwhile, Jericho is discovered following Ban and King on the journey to the Fairy King's Forest. In the forest, the fairy people call King a traitor because he left the Fairy King's Forest 700 years ago, his absence allowing the forest to be almost entirely destroyed, while Ban is called the new Fairy King because he planted the last Sacred Tree seed that Elaine entrusted to him. Diane and Elizabeth admit to each other their love for King and Meliodas respectively. Meanwhile Gowther, believing he needs to experience love, alters Guila's memories so she forgets Zeal is her brother and thinks Gowther is her boyfriend, while Zeal, who Gowther considers a barrier between himself and Guila, loses his entire memory. Gowthers manipulation of their memory enrages Diane and she attacks him without hesitation.
Diane defeats Gowther. The Ten Commandments discover the remains of a magical explosion and conclude it was caused by Meliodas. Gowther restores Guila’s memories. Merlin temporarily returns Gowther to his original form, a 6 inch tall wooden doll, and gives him to Slader for safekeeping. Sensing a magical disturbance Merlin transports the Boar Hat to Camelot which is being attacked by a giant Golem which Meliodas recognises as Albion, a magical weapon created by the Demon Clan, which awoke after it sensed the return of the Commandments. Meliodas breaks the sword he received from Liz so Merlin gives him back his sacred treasure, the sword Lostvayne, which she had retrieved from the pawn shop he sold it to. With Lostvayne returned Meliodas destroys the golem instantly. In the Fairy Kings Forest King and Jericho are surprised when the trees begin growing rapidly, which they realise is due to Ban feeding the forest with his blood which contains water from the Fountain of Youth, something the other fairies reveal he does every few years in honour of Elaine’s memory. The forest is suddenly attacked by another Albion Golem.
As the Golem begins destroying the forest King battles to protect the new Sacred Tree. Gerheade, the traditional servant of all past fairy kings, refuses to acknowledge Ban as king of the Fairies. She is surprised when Ban agrees with her, as he never wanted to be king. But, rather than let him leave the forest, she attacks him, knowing that he would risk himself to protect Elaine’s body and traps him, intending to use him as the new Fountain of Youth. King fails to stop the Golem and is injured while the Sacred Tree is destroyed for a second time. Gerheade prevents King from attacking the Golem while the other fairies rush to fight it, sacrificing themselves so King can escape, for as long as King is alive the forest will always grow again. King, however, refuses to flee, gaining strength from his desire to protect the forest, fairies, and even Ban he succeeds in transforming Chastiefol into its True Spirit Spear form and destroys the Golem. Back in Camelot as they recover from destroying the Golem they are attacked by Galand, one of the Ten Commandments, who is at least several times stronger than Meliodas.
Merlin notices that Galand’s magical abilities are still sealed away, giving hope they can defeat him. Ban uses his blood to restore the Sacred Tree and to heal King. Ban threatens to kill Gerheade if she ever tries to harm Elaine’s body again and then departs with Jericho, refusing King’s request to re-join the Sins. Galand easily overpowers both Meliodas and Diane. Merlin attempts to trick him with a lie, but Galand, who is the Commandment of Truth, cannot be lied to and Merlin is turned to stone. Meliodas is forced to unleash his demonic power. The other nine commandments realise that the magical energy once abundant in the earth has been absorbed into the souls of living beings, and by absorbing human souls; they can recover their magic. Despite his increase in power, Meliodas is killed by Galand, Slader is impaled, Diane is wounded and Merlin’s statue is smashed. Galand departs, sparing Arthur, Elizabeth and Hawke. Gowther arrives in his human form. Gilthunder comes across a village where a Red Demon is harvesting souls for the Commandments. He destroys the demon, returning the souls to their humans. Before they can celebrate the village is attacked by a Grey Demon. Hendrickson suddenly appears to help Gilthunder defeat the Grey Demon.
Hendrickson reveals that Dreyfus is still alive. Hendrickson defeats the grey demon using a druid spell previously used by Gilthunder’s father, Zaratras. Hendrickson reveals that 10 years ago he and Dreyfus investigated the destroyed country of Danafor. They encountered a demon named Flaudrin who possessed Hendrickson. Unwilling to kill Hendrickson’s body, Dreyfus was forced to allow Flaudrin to live inside Hendrickson and eventually to murder Zaratras and frame the Sins as revenge against Meliodas who had defeated him 16 years ago when Flaudrin destroyed Danafor and murdered Meliodas’ lover, Liz. Meliodas awakens in Camelot with everyone healed except for Merlin. Gowther had tampered with Galand’s memory so he believed he killed everybody. Merlin, who had transferred her soul into her sacred treasure, the magical crystal Aldan, explains that all the Sins combined have a combat level of 22,000, while Galand by himself had a level of 26,000. This means if the Commandments succeed in regaining their magic their combined combat level will be over 300,000. Meliodas decides to locate the final Sin, Escanor, the Lion’s Sin of Pride. King arrives wanting to see Diane, only to find Diane has amnesia and no longer remembers who anybody is, including King.
Diane’s memory continues to disappear, suggesting her memories are being tampered with. Diane disappears and Gowther admits that after his fight with Diane over erasing Guila’s memories, he decided to erase Diane’s memories as part of an experiment. He fails to understand why everyone is upset with him. King throws him through a wall. The sins suspect Diane may be heading back to the giants village, Megadoza. A flashback to 16 years ago shows Diane with fellow warrior giants Matrona and Dolores. Dolores is scared of war and wants to leave Megadoza but is too afraid of humans. Diane encounters Meliodas for the first time and realises people exist who are friendly to giants. Diane rushes to tell Dolores but finds she has been killed after being sent on a dangerous mission by Matrona. Diane realises she wants more from life than constant war despite Matrona planning to train her to become chief. Matrona and Diane are hired by Holy Knight Gyannon to fight a war. However Gyannon betrays them, intending to kill Matrona and gain fame. Matrona kills Gyannon but is wounded by a poisoned spear. She urges Diane to keep growing stronger then dies in her arms.
Merlin senses that the Commandments are close to Diane. Diane believes Matrona has only just died and is rushing back to Megadoza to tell her people, not realising 16 years have passed. Galand and fellow Commandment Monspeet come across Diane. As Monspeet is about to drain the magic from Diane she is saved by another giant resembling Matrona who knocks Diane unconscious and hides from the Commandments. Monspeet senses what he thinks is Meliodas and fires a powerful spell at him from hundreds of miles away. However the spell is swallowed by Hawks Mom, negating its power and shocking everybody who had no idea she could do that. The giant escapes with Diane but is now injured. Merlin reveals that many years ago she took most of Meliodas power as a precaution against him losing control and hid it in Istar, the Land of Druids, but now decides he needs it back. They reach Istar and meet the Druid leaders, twin sisters Jenna and Zaneri. Meliodas must pass a trial to regain his power. As Meliodas begins the trial he suddenly awakens in the past in Danafor before it was destroyed. He is greeted by Liz, who died years ago.
Ban and Jericho are searching for methods to revive the dead and are pointed to Ravens, the city of thieves, where a dead body was supposedly revived. As Meliodas undergoes his trial Elizabeth is given a trial to make a flower grow from a dead seed with healing magic. Meliodas is forced to relive happy memories of Liz and of the day she died, hundreds of thousands of times in the hope he learns to control his emotions. If he does not, the emotional strain on his heart will kill him. King and the others realize Hendrickson has been nearby the whole time, training with Gilthunder and his knights, and is human again. Rather than control his emotions Meliodas instead resolves to never again let a friend die like Liz and passes the trial. In Ravens Ban encounters a were-fox accused of raising the dead, though the fox insists itis untrue before collapsing. Ban recalls that as a child he was imprisoned for theft and met a man named Zhivago who offered to help him escape.
Hendrickson tells the Sins he is ready to accept whatever his punishment is, just as soon as they defeat the Commandments. In Ravens Ban remembers the second time he met Zhivago. Ban had been kidnapped by slavers but Zhivago saved him and taught him how to steal. Eventually Ban attempted a burglary by himself but was caught. The were fox, who is revealed to be Zhivago, was forced to choose between saving Therion, his were fox son, from hunters, or to save Ban. He chose to save Therion, who ended up being killed anyway, and he spent 30 years believing Ban also died. Ban reveals who he is and does not blame Zhivago for leaving him. Hendrickson, the knights and the other Sins are made to enter the training cave. Gilthunder and Howzer fight a dragon together. Gowther and Arthur battle a golem suit of armour. King enters the cave with Meliodas, who asks King to understand that Hendrickson did what he did because of Fraudrin, who Meliodas reveals is one of the Ten Commandments. King confronts Meliodas about being a member of the Demon Clan and questions whether Meliodas is even on their side before attacking him.
Meliodas and King continue to fight each other. Ban talks to Zhivago and Jericho about his immortality and Elaine who he is determined to resurrect. Elsewhere in a cemetery a dead body digs its way out of its grave. Ban admits to his greatest regret, that he once tried to kill Meliodas in exchange for Elaine’s resurrection, and he hates himself even though Meliodas forgave him. Zhivago urges Ban to reconcile with Meliodas and tell him his real feelings. Zhivago then dies peacefully. King accuses Meliodas of being the Commandments secret ally. Meliodas questions whether King is a king at all, as the previous fairy kings had prominent wings, whereas King has none. The fairy twins stop them fighting before they can hurt each other. Meliodas promises to reveal the truth, eventually, so King decides to continue watching him. As they leave King senses sudden pain on his back. Meliodas regains his old power and has Merlin teleport him to the Commandments. He is challenged by Galand but with his power returned Meliodas easily beats him. He then gives the Commandments a warning, if they restart the war they lost 3000 years ago, Meliodas and the Sins will crush them.
The Commandments spread out to towns and cities across the country to recover their magic. Meliodas plans to defeat the Commandments one at a time rather than together. Everyone completes their training except for Elizabeth who fails to make the dead seed grow. Jenna and Zaneri, who has an unrequited love for Meliodas, discuss how Elizabeth is the reincarnation of Liz, Meliodas’ deceased lover, and how Zaneri sabotaged Elizabeth’s training to try and separate Meliodas from her so he would never realise who she really is. Meliodas announces they still need to find Escanor, the Lion's Sin of Pride, who is supposedly even stronger than Meliodas. The resurrection of the dead is revealed to be the work of Melascula, the Commandment of Faith, who is resurrecting and corrupting dead souls to form an army. One of the souls she summons is Elaine, who also returns to life corrupted, and she immediately starts searching for Ban. King senses Elaine’s revival and rushes to the forest, refusing Meliodas’ help as he still does not trust him. As Ban and Jericho bury Zhivago they are attacked by a corpse, which Ban easily destroys. Elaine appears and reunites with Ban and they kiss passionately.
A revived but possessed Elaine attacks her lover, Ban, as Melascula and Galand of the Ten Commandments emerge to destroy those in their path.
Jericho carries severely injured Ban and Elaine on her back while escaping from Galand and Melascula before stumbling into a bar deep inside a cave.
Gloxinia and Drole of the Ten Commandments lure strong fighters to Vaizel by offering to grant any wish to the winner of their Great Fight Festival.
An elaborate trap-laded deadly maze constructed by Drole begins to weed out the weaker challengers that have gathered for the Great Fight Festival.
The surviving challengers of the Great Fight Festival are split into pairs and made to face each other in a tag team battle.
King and Diane continue their battle against opponents summoned by Gloxinia and Drole. Escanor refuses to fight against his fellow ally Gowther.
As Escanor's battle with Gowther comes to a dramatic end, Meliodas must now face the remaining Ten Commandments in an all-out battle.
With Meliodas rendered powerless, Ban asks to be transported back so he can aid him in his fight against Estarossa and the other Ten Commandments.
While caring for Meliodas, Elizabeth encounters a former Great Holy Knight thought to have perished long ago by the hands of Dreyfus and Hendrickson.
With Ban and the Holy Knights paralyzed by Estarossa’s powers, Escanor shows up to thwart the fall of Liones at the hands of the Ten Commandments.
As the situation inside Liones Castle becomes even more dire, Merlin, freed from Galand's Commandment of Truth, reappears in her former form.
Determined to kill his former leader for betraying their clan 3,000 years ago, Fraudrin takes on a newly resurrected darker, more powerful Meliodas.