In a flashback, before Ban had become a member of the Seven Deadly Sins, he was a bandit who sought the Fountain of Youth, a liquid sap following from the Fairy King's Forest's Sacred Tree, which sustains the entire forest. Although initially repeatedly thwarted by Elaine, the Holy Maiden and Protector of the Fountain of Youth, she soon realises that Ban is different from other humans that attempt to drink from the Fountain due to greed, who instead simply wants to live long enough for something good in his life to happen. In the present, Meliodas, Elizabeth, Hawk, Diane and Ban make plans to go to the Capital of the Dead, catching a glimpse of King, whom Ban does not recognise at first due to his tremendous loss of weight. With help from two siblings, the six, trailed by King, find the entrance of the Capital of the Dead entrenched in a field of flowers, accessible only by the living who share a priceless memory of the dead.

Ban's memory of Elaine unlocks the door, allowing them to enter the Capital of the Dead, an empty world surrounded by neon green crystals. Glimpsing Elaine, Ban runs towards her but is petrified by King in the process, who seeks revenge on him after overhearing rumours that Elaine's death was caused by him, and revealing that she was his younger sister. Ban has another flashback where he and Elaine grew close and fell in love, but the two were severely injured while confronting a red demon rampaging the Fairy King's Forest. Elaine then sacrificed her life by ingesting the Fountain of Youth and feeding its power to him through a kiss, allowing Ban to attain immortality and kill the red demon. In the present, a Holy Knight named Guila enters the Capital of the Dead, battling Meliodas and Diane while Elizabeth and Hawk run away to safety. Elaine, first unseen to King, releases Ban from petrification with a kiss, allowing him to join Meliodas and Diane in the battle against Guila, while Elaine tells King to understand Ban and save him. After being informed by Elizabeth and Hawk about the ongoing battle, King executes a surprise attack from behind through Ban to strike Guila.

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 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.

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