One year after How to Train your Dragon 2, Hiccup and Astrid are a happy couple until, one day, their relationship is threatened by a new and attractive dragon rider, Ryder, who comes to Berk and develops an interest in Astrid. Astrid is intrigued by the newcomer, however Hiccup is suspicious of him. Hiccup's newfound emotion of jealously influences him to attempt to keep Astrid away from the new rider, but he finds himself making the wrong decisions. And making things worse. Plans go wrong, people get hurt, and danger looms ahead.