Every decade, Aurvandil's Fire arrives to Berk, inviting a ghostly dragon, the Flightmare, to the village. Astrid, with the help of Hiccup and Fishlegs, is determined to defeat this dragon, which seemingly "froze" her uncle with fear, in order to restore her family name. Meanwhile, Snotlout tries to get into Ruffnut and Tuffnut's exclusive anti-Flightmare bunker.
- Bioluminescence Is Cool: Any dragon that eats the glowing algae starts to glow itself. The Flightmare, Toothless, Stormfly, and Meatlug are all glowing by the end of the episode.
- Bullying a Dragon: Snotlout laughs at how Fearless Finn Hofferson was apparently a coward, to Astrid.
- Butt-Monkey: Snotlout becomes this for the episode, first after mocking Astrid's uncle, and then after trying (and failing) to get into the twins' bunker.
- Call a Rabbit a "Smeerp": What we know as an Aurora Borealis/the Northern Lights is called Aurvandil's Fire by the Vikings.
- Comically Missing the Point: Hiccup tries to flirt with Astrid saying he doesn't just think about dragons. Her response is if he's saying that with a straight face.
- The Dreaded: The Flightmare's paralyzing mist gives it a massive advantage over its enemies. The Vikings, who see others freeze while trying to fight it, believe Fear Is the Appropriate Response is in play.
- Dude, Not Funny!: In-universe Astrid nearly beheads Snotlout for laughing about her uncle.
- Family Honor: The main conflict besides the Flightmare itself; Astrid's uncle, Fearless Finn Hofferson, was killed after falling victim to the Flightmare's paralyzing mist. To the other Vikings, it looked like he'd frozen in fear, and they renamed him "Frozen" Finn Hofferson. Astrid's motivation throughout the episode is to regain her family honor by killing the Flightmare.
- Getting Crap Past the Radar: Hiccup rescues a paralyzed Astrid and holds her in several awkward positions.
- Hoist by His Own Petard: Astrid thinks to have the dragons drink the river algae, so that their fire will paralyze the Flightmare and scare it off long enough to allow Fishlegs and Meatlug to redirect the river.
- Impossible Task: Snotlout has to accomplish a series of them to be allowed into the bunker. Surprisingly, he succeeds.
- Laser-Guided Amnesia: The Flightmare's Breath Weapon causes this in the victims as a side effect.
- Leeroy Jenkins: Both Fearless Finn and Astrid have the exact same strategy of charging straight at the Flightmare, allowing it to defeat them just by breathing in their direction. Astrid is saved from certain death by Hiccup; Finn is not so lucky.
- Moving the Goalposts: Tuffnut starts adding items to Snotlout's requirements for being allowed into the bunker. Snotlout calls him out for it.
- The Paralyzer: The Flightmare's Breath Weapon is this, being absorbed through the skin and causing victims to be unable to move for about a minute.
- Ship Tease: Hiccup attempts to flirt with Astrid.Hiccup: Well, you know Astrid... training dragons isnt the only thing I think about.Astrid: Are you actually saying that to me with a straight face?
- Troll: The twins to Snotlout.
- Yank the Dog's Chain: Snotlout doesn't get into the bunker until the end, and everyone else has left to celebrate the victory over the Flightmare.