Hiccup, Merida, Rapunzel, and Elsa's kingdoms(/island-dom) are besought with a mysterious and terrifying plague. Their people have become violent, and cannibalistic, doing and saying anything to feed on each other. The infected look entirely normal, Except for their eyes; Black as the deepest sea.
Hiccup finds a ship from Corona (Rapunzel's Kingdom) full of the infected. After attempting to make peace with the men, they attack, and he is forced to flee. He goes to consult Rapunzel about this, only to find her entire kingdom destroyed and empty. Her Castle under siege of the Infected, Hiccup must find a way to save the innocent people of Corona, while also warning his people, Merida, and Elsa.
Meanwhile, Jack is transported through time and space to a land full of fear and plague. To find the man who sent him there, he must work with Hiccup, Merida, Rapunzel, Anna, and Elsa, to cure the plague and find a way back home. But this will become infinitely harder, seeing as Jack has been turned into a Human.
Elsa and Hiccup have been enemies since like forever. They both hated each other but deep inside they had feelings for each other. Hiccup is the popular bad boy of school,while Elsa is the schools biggest nerd. One day Rapunzel Elsa's only friend get's invited to a high school party which is hosted by the the most popular guy in school Jack Frost. At the party Elsa and Hiccup meet up once again and they both get drunk and start making out which leads to other things. In the morning Elsa realized she made a huge mistake and finds out that there is also another mistake on the way. Elsa makes the decision to run away from Berk and move back to Arrandelle alone. After five years Elsa comes back, but not alone. She comes back with a young boy. Will Hiccup find out and remember what he and Elsa did or will he just ignore her and care more about popularity ,find out in My Best Mistake.