Third episide if Defenders of Berk.
Astrid has devised a new training exercise for the Riders. They are understandably hesitant given Astrid's previous exercise "hand to claw combat" but are relived to learn that the exercise is to trek across Dragon Island. However their relief disappears when they learn that they must do so dragon-less and at night. The exercise is meant to develop the riders stealth abilities which Hiccup supports.
The group arrive at Dragon island at night, Hiccup has Tuffnut take the dragons to the other end of the island so as to avoid getting unwilling help from them. On his way to the other side of the island Hiccup investigates a campsite and learns that it belongs to Dagur the leader of the Berserker Tribe. Dagur has been on the island for an unknown amount of time studying and hunting dragons.
Asking Hiccup about what he has been up to ever since he "saved him from" Toothless. Hiccup lies and informs Dagur that Toothless got away before he could kill him leading Dagur to conclude that Hiccup is hunting the dragon down to kill it and decides to help. While Chasing after Toothless Hiccup and Dagur meet up with the other riders and Hiccup instructs them to get their dragons and get off the island.
Hiccup attempts to get Dagur to leave Toothless alone trying to convince him that he on his own won't be enough to bring the dragon down, to this end Dagur reveals that he is pushing Toothless to the shoreline in order to have the entire Berserker army shoot him dead. Once finally at the shore Hiccup is forced to reveal that he is a dragon rider in order to save Toothless and uses His shield to fend off Dagur.
Hiccup also informs Dagur that he had best be careful before he even thinks about attacking Berk, as that means dealing with the dragons as well. While Hiccup reunites with the rest of the riders Dagur reunites with the rest of the Berserkers and informs them that they are going to war with Berk, and that Hiccup and Toothless are his and only his.