In the film, Astrid interrogates Eret for the location of Drago Bludvist. She does this by using Stormfly to play with him, dropping him from vast heights and catching him at the last second. To keep Eret from escaping, Astrid leaves Eret at the mercy of her dragon, who proceeds to sit on him like a chicken and its egg, claiming that "it is unwise to take a toy away from a deadly nadder." When Drago order's Eret's death, Stormfly comes in and saves him. This inspires Eret and compells him to side with the riders, helping them escape the guards and free their dragons, Eret thanking Storfly personally.
Much like with Toothcup, this pairing often portrays Stormfly not as a dragon, but as a human - or at least a humanoid - to better flesh out their relationship.
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