Part 1
Following their coronation as king and queen of Liones, the demon Meliodas and goddess Elizabeth have a son named Tristan, who inherits both respective abilities of his parents. During a sparring match with his friend Lancelot, the half-fairy son of Meliodas's comrade Ban, the ten-year-old Tristan is briefly overtaken by his demonic impulses and seriously injures Lancelot, who goes missing shortly afterward. Ashamed and afraid of losing control of himself again, Tristan seeks out less violent applications of his powers, which strains his relationship with his parents.
Four years later, Elizabeth is struck with a fatal illness due to a curse placed on her by Priest, a servant of the former Holy Knight Deathpierce, who seeks vengeance for the injustices he suffered from the Demon and Goddess Clans fourteen years earlier. Trying in vain to heal Elizabeth with his goddess powers, Tristan senses Priest and rushes out of Liones to save Elizabeth. Meliodas's comrade Gowther places a tracker on Tristan and contacts Ban, Diane, and King, their companions from the disbanded Seven Deadly Sins. Meanwhile, Deathpierce begins capturing demons, giants, and fairies to combine them into monstrous chimeras with his mystical Chaos Staff, intending to conquer Liones with them.
Tristan encounters a fairy who has saved three non-human children from the Empties, Deathpierce's army of magical automatons. Despite the fairy's hostility towards Tristan, the two join forces and follow the children's captured parents to Deathpierce's domain of Edinburgh, where they fend off Deathpierce's army and rescue the parents. When Tristan is nearly killed while struggling to control his demon instincts, the fairy saves him and assumes a human-like form, revealing himself to be Lancelot.
Part 2
Tristan confronts Lancelot for disappearing, but Lancelot remains bitter over their childhood accident. Faced with another wave of Empties, the two bypass them on an ice bridge conjured by a knight with a star-shaped visor, leading them into Deathpierce's castle. While Tristan and Lancelot battle Deathpierce and Priest, Diane throws Meliodas, Ban, and Gowther directly to Edinburgh, where they eliminate Deathpierce's army. After Tristan defeats Priest, Elizabeth fully recovers and arrives with Diane and King shortly afterward.
Tristan and Lancelot follow Deathpierce as he retreats into another dimension. Anguished over his unfulfilled revenge, Deathpierce is transformed by his Chaos Staff into a powerful abomination. Lancelot encourages Tristan to stop holding himself back, explaining his bitterness comes from Tristan's refusal to continue their sparring match. Now trusting Lancelot's ability to stop him, Tristan willingly unleashes his demonic instincts and defeats Deathpierce, who returns to human form. Lancelot then subdues the mindless Tristan and destroys the Chaos Staff, knowing that Deathpierce received it from Arthur Pendragon in support of his ambitions.
The staff's destruction returns Tristan, Lancelot, and Deathpierce to Edinburgh, where Tristan reunites and reconciles with his parents. Ban sees Lancelot leaving the castle alone and recognizes the ice bridge as the work of Jericho, Lancelot's missing caretaker. The others return to Liones while Lancelot walks off with Jericho following him in the distance.
