Sins of the Past is the thirteenth episode of Dragons: Race to the Edge, Season 5. It was released on August 25, 2017.
After Heather is lured to the Northern Markets as a trap, Hiccup flies to find her, but when he arrives too late, Windshear is captured by Krogan. 
Snotlout is sent to the Northern Markets to fetch some chicken feed for the twins, and ends up being talked into giving up his tunic and body armor to get the most expensive feed called Archipelago Gold. Heather and Johann happen to be at the markets too, to meet with the man who says he saw Oswald alive. While Heather is in a tavern taking with the stranger, named Harek, Snotlout runs into Johann who is outside waiting for her. Heather soon learns however that Harek is a phony and is forced into a conflict with Krogan. Windshear senses something wrong and alerts the two men to go to her. They help her fight the Hunters off.
Meanwhile on Berserker Island, Dagur, Hiccup, and Fishlegs are resting up from their recent trip to Vanaheim when a Berserkian guard welcomes Dagur home. When asked where Heather is, the guard says that she left with Johann for the Northern Markets to meet a man who says he recently saw Oswald alive. The two Riders and Dagur are shocked, and concerned for her safety since they know Oswald is dead, immediately saddle up and take off again to go meet her. At the markets, Dagur is still learning to be sensible with witnesses as they question merchants about where Heather is. The one who sold Snotlout the chicken feed finally tells them of the fight, and which way they all few off.
Heather, Snotlout, and Johann are still being chased by the Dragon Flyers, and Heather and Johann are knocked off of Windshear. Hookfang catches them, but Windshear is caught in a net and carried off as ransom by the Flyers. Heather insists they go rescue her, but Snotlout says that Hookfang is carrying too much weight and needs to rest first. Once landed, Krogan reappears and announces that they can save both their own lives and Windshear's if Heather gives him the Dragon Eye lens on her belt. Under Johann's advisement, saying it's just a "thing", not to be compared to a life, Heather agrees. She meets Krogan on a sea stack and they make the trade for the key to Windshear's cage, but when taking off, Krogan knocks Windshear's cage into the sea. Heather jumps in to unlock the cage but finds the key is a fake. Just as she's accepting that this is it, Dagur and Sleuther dive in and carry the cage out of the water. Hiccup and Toothless also appear and Toothless blasts the cage open with a plasma blast. The Riders are shocked and worried that Krogan and Viggo have Heather's lens, but Fishlegs is thankful that they have both her and Windshear back and they're alright. Back at the Edge, Snotlout gives the twins the chicken feed, which had popped into pop corn during the fight with the Flyers, and Dagur finally gives Heather the letter their father wrote for her, where she also learns that he's dead.
However, at wherever the Dragon Hunter Headquarters are, Viggo inserts the newest lens into the Dragon Eye, and Johann enters, saying how much he hated kissing Stoick's boots and befriending Hiccup for years in order to build up trust, and then risked everything trying to get the lens, for they hope it will lead them to the "King of all dragons ".
- Snotlout is said to be the creator of popcorn.
- Johann said that the lens was going to help him find the "King of all dragons" and in the second movie, Valka said that the Bewilderbeast is the "King of all dragons" so it will most likely be a Bewilderbesat dragon.
- When Dagur, Hiccup and Fishlegs leave to find Heather, the rest of the riders sans Snotlout were staying on the Edge. After Dagur saved his sister from drowning Astrid was suddenly present in the scenes as well. It's possible however that Hiccup stopped by the Edge first and invited her along.
- Oswald spoke of Heather in his letter as if she was still with them before he disappeared and that is why he asked Dagur to look after his sister. This contradicts with Heather's tragic backstory of being set adrift as a toddler.
- Oswald also referred to Heather by the name Heather's foster parents gave her.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut Thorston
- Andree Vermeulen as Ruffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- Mae Whitman as Heather
- David Faustino as Dagur the Deranged
- Hakeem Kae-Kazim as Krogan
- Alfred Molina as Viggo Grimborn
- Michael Goldstrom as Johann
- Jeff Bennett as Harek