The episode opens in the days of Astrid Hofferson's childhood during an attack on the village by the legendary Flightmare during the time of 'Alvindale's fire' (more accurately the aurora borealis), a legendary dragon known for it's phantom like glow and ability to freeze even the most fearless of Vikings stiff with fright. Astrid's uncle Fearless Finn learns this first hand when he tries to confront the dragon himself.
In modern times the time of Alvindale's fire is fast approaching and Astrid has been training herself rigorously in preparation for the Flightmare's return to restore her family name. Hiccup however proposes that they find and study the Flightmare before he invades the village to find some way of redirecting him. According to Fishlegs the Flightmare always takes one particular route into the village, following one specific river.
Finding the Flightmare along the path Astrid attempts to confront it on her own, and for her troubles is left paralyzed like her uncle. However Hiccup realizes that the Flightmare relies on some sort of paralytic spray to defend himself. Further more they also discover that the Flightmare eats a type of rare algae found in the river that gives the Flightmare and any dragon that eats it a glow in the dark.
The reason why the Flightmare always attacks the village is because the river leads into the village and thus the highly territorial dragon believes that the villagers also eat it. However if they redirect the water to the ocean then the dragon will fly away from the village and never come back. To this end Fishlegs comes in with Meatlug and Stormfly and together with Toothless they dig a channel to the ocean.
With the Flightmare gone for good Hiccup brings the good news back to the village, with the added news that, just like all the Hoffersons, Finn was indeed fearless.