Mildew is a the secondary antagonist in Dragons: Riders of Berk.
In Animal House, he was not seen but mentioned. According to Mulch, after a storm known as "Blizzard of Olaf," it took some Vikings a week to dig an unlucky Mildew out of the ice (and the rest of their lives to wonder why they bothered.)
In In Dragons We Trust, Mildew frames the dragons for destroying things around Berk, successfully causing them to be banished. He makes it look as though the dragons have stolen shoes, wrecked the great hall, and finally destroyed the armory. He used fake dragon props to accomplish his goals. Only Hiccup manages to see these devices. Unfortunately, Mildew then disposes of the evidence by throwing it out to sea.
In Alvin & the Outcasts, Hiccup tries to find the evidence Mildew had disposed of in the last episode, but to no avail. When Alvin the Treacherous arrives and searches for the "Dragon Conqueror" (Hiccup), Mildew tries to tell him but is knocked out by Astrid. At the end of the episode, he was furious to see the dragons once more- the village having accepted them back after the dragons helped them drive off Alvin's men, but was warned by Stoick, along with Hiccup saying that he may not have the evidence that proved he framed the dragons, but will not forget it, leaving a frustrated and disappointed Mildew.
In Dragon Flower, Mildew was seen buying mysterious items from Trader Johann, in exchange for cabbages. The next day, all of the dragons become ill for an unknown reason. Although everyone throws out what they bought from Johann, the dragons are still sick. Fishlegs tells them that the cause is the blue oleander flower, which is poisonous to reptiles which means poisonous to dragons. Tuffnut says that he saw Mildew plant them at night. When Stoick, Hiccup and Astrid interrogate Mildew, he pretends to not know of the flower's "special properties." Hiccup, Stoick, and Gobber board a ship to find the Scauldron, a dragon that feeds off the oleander, and whose venom can be used as an antidote. Mildew is dragged onto the ship, much to his displeasure, because it was his fault the dragons were sick. When the Scauldron is found, Mildew is threatened by Stoick to wind up with the bucket that's supposed to be used to get the venom or Stoick will throw him overboard. However, before the venom can be drained, the sea dragon escapes, but not before biting Mildew's butt, trying to get the flower that was stuck to his pants. Gobber takes Mildew back to his workshop, and uses a large needle-like object to pull the poison out of the old man, much to his discomfort.
In When Lightning Strikes, lightning from thunderstorms strike the metal perches recently set up for the dragons, and seemingly follow Toothless. Mildew accuses Toothless of causing lightning strikes due to his status as 'the unholy offspring of lightning and death' (according to the Book of Dragons), and rallies up the citizens to banish the Night Fury. When Hiccup proves to everyone that the lightning is really attracted to metal, Stoick suggests taking the iron statue of Thor, built to appease the thunder god (but failed), and placing it high up in the mountain, where the gang puts it directly outside Mildew's house. When the next storm comes, the lightning goes through the statue and into Mildew's house, shocking him repeatedly.
In We Are Family Part I, it is revealed he set a trap for Hiccup and Toothless by forging a map to an island with Night Furies. It is also revealed he has joined the Outcasts. His method of trickery was personal and devious. He used Hiccup's love for Toothless as bait, and then lured them both into a trap. Not only this but he ensured that none of the other kids would come. His actions alone led to Hiccup and Toothless being captured by Alvin and his men. Upon seeing Hiccup captured, he openly mocks him for falling into a trap set by a silly old man. It is unknown how long Mildew has been in league with Alvin, but it might help explain how convenient Alvin's arrival to the island was in Episode 6.
In We Are Family Part II, Mildew begins to regret what he has done after Alvin locks him up and takes away Fungus to be cooked as his final meal. He then helps Hiccup escape, and both Hiccup and Mildew work together and befriends a Deadly Nadder with Hiccup's help. "Unfortunately" Mildew is dropped from the Nadder that Gobber is riding, and falls back onto Outcast Island as the gang is making their escape Later it is revealed that he and Fungus were all putting on an act, so that he could learn all the dragon training secrets from Hiccup. He said to Alvin, "How did I do?" Mildew truly is a traitor to Berk. He then begins to tell Alvin the information he learned from Hiccup. With that information, Alvin was able to train a Whispering Death, the last known aired episode for now.
In Live and Let Fly, Mildew came up with the idea to plant a cluster a Whispering Death eggs in a underground network of tunnels so that they would hatch and destroy Berk. He and Savage oversaw this plan being put into action while Alvin distracted the dragon riders but what he was unaware of was that one of the eggs was a Screaming Death egg.
In Worst in Show, Mildew, Alvin and Savage infiltrated Berk to investigate what happened with their previous plan, which, unknown to them, resulted in the creation of the Screaming Death. Mildew managed to save himself from Alvin's wrath by suggesting they use the tunnels to infiltrate the Dragon Academy to kill, spy and kidnap and, later on, helped the other two to kidnap Meatlug, who had been neglected by Fishlegs at the time. When tried to make their escape, he was knocked unconscious by Butt and Head, who had been trained by Ruffnut and Tuffnut, and only came around to see that they were all drifting on wreckage. Alvin immediately knocked him unconscious when he tried to make them look on bright side.
In A View to a Skrill Part II,
In Cast Out Part II,He was seen feeding a Whispering Death as he showed Hiccup where the eggs came from that were planted under berk and asking if there was a giant red egg [Screaming Death egg] and later seen fighting the army of Berserkers