During the week of the Midnight Sun where the sun doesn't set for a week, Hiccup must deal with sleep deprivation and a dangerous new dragon.

The Riders accept an invite to Berserker Island, where their reunion with Heather, Dagur and a young familiar face is interrupted by a sudden revolt.

An unlikely ally helps Hiccup uncover Singetail behaviors that can be exploited to conquer the Flyers. An overconfident Krogan makes a strategy error.

After learning several Flyers followed Heather to Vanaheim, the Riders must stop them from reporting the sacred island's location to their leader.

The Riders race to the home of the true King of Dragons. But the Flyers are already there, and they soon come face to face with the legendary creature.