Hiccup, becoming the leader of the island of Berk did not leave his research. One day, together with Eret and Astrid, he opens a new island. Suddenly, an unknown dragon appears and attacks them. Astrid and Hiccup were able to fly away, but Еret was trapped and stayed. He meets, Eidis - the mistress of the island, which is on good terms with the dragons. Her personal dragon, it turns out Tara is a female night fury. Hiccup and Astrid return for Eret and also meet Eidis, who recognizes Hiccup, but does not say why. Together they travel to the island of Berk, where Eidis learned Valka, her mother. Hiccup is very surprised, but genuinely happy to have his sister back. Before returning to his island, Eidis participates in the race and wins with Hiccup.
A month later, Hiccup and Astrit fly back to the island of Eidis, but then notice that something is wrong. Among the ice spikes, they notice the weakened Eidis. Hiccup goes down to her, and Astrid flies away for help. Eidis, tries to warn her brother, but its too late. Dark alpha and Drago appear. Tara and Eidis are trying to fight him off, but it's no use. Taking advantage of the situation, the dark alpha freezes Hiccup, and then hypnotizes Tara. At this point, eret appears, flew to the rescue. Drago sets Tara on him, but Eidis at the last moment manages to push him away and she gets hit. Drago and his army are going to Berk. Again toothless releases Hiccup from the ice prison. On the ship, Еret deals with the guards, saving himself and Eidis. Hiccup, learning what happened to Eidis, goes with friends in pursuit of Drago. Meanwhile, eret, Eidis and several of Drago's men escaped on the boat. Eret tries to find out who likes Eidis, but not knowing that it is he himself, begins to get angry at this unknown. Here they come riders of Berk.