|“||When Dagur unexpectedly shows up on the Edge, Hiccup quickly scrambles up a plan to prevent him and Heather from discovering each other's presence.||”|
- Shattermaster makes his first appearance.
- Dagur befriends Shattermaster.
- Dagur supposedly dies after destroying the Dragon Hunter's Shipyard.
Returning to his hut after a training exercise in preparation for an assault on Viggo's shipyard, Hiccup shockingly discovers Dagur has returned. Dagur has visited to request Hiccup to lend him a dragon after realizing the immense difficulty of his mission to find Heather by boat. Hiccup initially refuses at first but changes his mind when Dagur plans to instead meet with the Riders. To avoid his presence known, Hiccup has him camp out at the outskirts of the island to await his training tomorrow. Shortly afterwards, Hiccup cuts into the Riders' conversation during dinner and asks their opinions on Dagur's sudden behavior during their encounter. The Riders reject the notion that Dagur is changing his ways, especially Heather, who vehemently emphasizes she'll kill him when she sees him. Realizing Heather is assigned to patrol the island, Hiccup reassigns her with Snotlout to do reconnaissance on Viggo's shipyard tonight, buying him time to deal with Dagur.
At dawn, Hiccup meets up with Dagur, anxious to ride a dragon of his own, having a name already for his future dragon, Shattermaster. But his expectations are ruined when Hiccup picks out a wild Gronckle for him to ride. Dagur reluctantly agrees to train with the Gronckle. As training goes on with flying, Dagur pushes Shattermaster too far and they get knocked back against Hiccup and Toothless. The incident results in Hiccup's Prosthetic Leg ruined, so he pauses their training and rides Shattermaster to retrieve his spare while Dagur and Toothless stay put. In the meantime, Dagur rides Toothless and fly through the forests, but Dagur is accidentally pushed off the saddle. When he touches the ground, he is suddenly surrounded by the Riders who have retrieved Hiccup's damaged prosthetic, accusing him of harm.
Dagur is taken captive and is brought into the arena as the Riders interrogate him for answers on Hiccup's whereabouts. Dagur tries to convince them he has done no harm to Hiccup but to no avail until finally, Hiccup shows up to finally clarify his new relations with Dagur. Dagur then explains to them that his training with a dragon is for the purpose of finding Heather. The Twins are willing to tell him she's with them, but Hiccup has Dagur distracted by assigning Fishlegs as his new instructor and continues their training. Afterwards, Hiccup proceeds with Astrid to discuss strategies on their war table that outlines the Dragon Hunters' shipyard. But Astrid instead brings up her suspicions on Dagur as a spy with his sudden presence on the time they're about to assault the shipyard. Despite being cautious with Dagur, Hiccup tries to justify his trust on Dagur after his own dragon trusted and let Dagur ride him in order to rescue Hiccup from the Hunters. When they prepare to leave, Astrid tries to ensure the secrecy of their plans by covering up their war table, but a single ship model falls and goes unnoticed.
Dagur's training shows excellency with Fishlegs and surprisingly with Astrid as well. Nonetheless, Astrid expresses her reluctance to Dagur, threatening him if he dares to betray them. Dagur continues to bond with the other Riders. Astrid reminds Hiccup the urgency that they have to finish Dagur's training or tell him her sister's presence. Unfortunately, Heather and Snotlout make an early return and in an instant Heather charges and launches his axe against Dagur, but narrowly stops, and instead lands a hard punch. Shortly after, Heather lashes out on Hiccup, for consorting with the enemy, and Dagur, reminding him of all the horrible crimes he committed, such as the murder of their father. Dagur pleas with Heather and the Riders that he is trying to change his ways but falls into deaf ears, and is accused as a spy. Dagur sorrowfully decides to leave but then comes across the Riders' model of the shipyard for inspection. Hiccup reveals their mission of assaulting the remote shipyard, claiming that the Hunters still believe the location is unknown. However, Dagur believes the seemingly ill-equipped shipyard is a bait for the Riders. Astrid then finds them, and regardless of whether or not he's a spy, decides to imprison him to avoid the risk of the plan being known to the hunters. Dagur complies but berates Hiccup, still firm on proceeding with their assault on the shipyard despite Dagur pointing out the warning signs of a trap.
Hours later, while preparing for their strike, they receive news of Dagur's escape with Shattermaster. Hiccup immediately orders the assault and race ahead Dagur, fearing he'll warn the Hunters. They managed to catch up, and Heather charges ahead. Using tactics taught earlier, Dagur is able to evade Heather and her dragon and makes it to the shipyard, while the Riders follow behind. Dagur emerges from the fog and attacks the ships. Alerted by his presence, the Hunters reveal their ambush just as Dagur suspected. From a distance, the Riders realize that Dagur was right all along. However, Dagur struggles to fend them off as the Hunters launch counterattacks from all directions. Dagur and his dragon bravely remain defiant and charge the hunters in a full frontal assault. Inevitably, the Hunters overpower them and they are seemingly killed as a result. Witnessing the tragedy, Hiccup attempts a rescue but reluctantly has to retreat with the Riders. Heather is the last to leave the huge damage inflicted on the shipyard, as the remaining Dragon Hunters cheer in victory.
With the mission over, Hiccup cleans up the last pieces of the shipyard model. The Riders feel ashamed of themselves after their mistrust of Dagur's warnings that resulted in his apparent death. Snotlout hopes that Dagur survived, but the others find it clear that he didn't. Heather retreats to her room unable to clear her mind after Dagur's actions. She stumbles upon a note written by Dagur prior to his suicide mission. The note expresses Dagur's commitment to not let her sister get harmed resolving him to face the trap alone. He tips them off with a suggestion to disrupt Viggo's dragon auctions that can significantly cripple Viggo's hold on the Hunters. He discloses to Heather that he lied about killing Oswald the Agreeable and that he went missing, explaining that he accepted the rumors that circulated that he did so that the Berserker Tribe can view him as a fitting chieftain. Lastly, Dagur conveys his deep guilt in his terrible actions but his full disclosure is his dying wish for peace and asks Heather to stay safe. Tears and rain droplets start to stain the note, as a crying Heather indicates her forgiveness for her fallen brother.
- This episode reveals that Oswald wasn't killed by Dagur, but yet missing.
- Though flying Toothless made him sick in "Enemy of My Enemy", Dagur seems to really enjoy flying him in this episode.
- Tuffnut reveals that the Twins' parents nicknamed them "Slow" and "Inaccurate".
- Tuffnut was "Slow" and Ruffnut was "Inaccurate."
- Hiccup explains that he felt guilty for Dagur's death because he gave Dagur a Gronckle to train as his dragon, rather than the faster, larger dragons that Dagur had wished for.
- Dagur considers himself a "real Rider" in this episode, as opposed to the other Riders.
- Snotlout appeared with Astrid, Fishlegs, Tuffnut, and Ruffnut when they found Dagur in the woods when he was really with Heather scouting for their mission. This was an animation error.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- Andrée Vermeulen as Ruffnut Thorston
- Mae Whitman as Heather
- David Faustino as Dagur the Deranged
- Night Fury
- Deadly Nadder
- Monstrous Nightmare
- Hideous Zippleback
- Skrill (mentioned)
- Hiccup's Prosthetic Leg
- Toothless' Saddle and Tail Fin
- Astrid's Old Axe
- Astrid's Kransen
- Heather's Axe
- Heather's Horn
- Heather's Dragon Eye Lens
- Dagur's Dagger
- Dragon Hunter Weapons
- Dragon Hunter Ships