How to Train Your Dragon was originally a book series that got made into a movie trilogy and a tv show. It is centered around a boy named Hiccup and his dragon, Toothless. This is around the time of Vikings, without modern technology. Toothless is the last dragon of his kind, a Night Fury. There were many tales about them being the spawn of lightning and death itself. All of the Night Furys were hunted to extinction except for Toothless who was captured and trained by Hiccup in the first movie. 

In the third movie, all of the characters are older and the Vikings live peacefully amongst the dragons. Almost every Viking in their village has a pet dragon that they ride, take care of, and train. They have always had to deal with other people outside of the village trying to capture dragons and kill them or use them for labor. Hiccup and his friends have continued to save these dragons from these people and bring them back to Berk, their village. However, lately, they have been saving too many dragons resulting in an overpopulation of dragons in the village, making them a bigger target for trappers. 

Hiccup’s friends realize that they need to have a more secure area to keep them but Hiccup just wants to keep saving them. They end up going on a flight around the island and see a Light Fury. A Light Fury is like a Night Fury but instead of being black, they are white with blue shimmers on the scales. Toothless is intrigued by her and just wants to follow her and be with her because Furries mate for life. As the movie continues, Toothless continues to become more infatuated with her. He even escapes Hiccup to be with her and fly around. 

At this point in the movie, Toothless becomes restless without her. He even ends up following her to “The Hidden World” which is a place where dragons of all kinds live without having to worry about humans. Toothless being the last Night Fury ends up going to the middle and becoming the alpha of the dragons. All dragons bow down to him and listen. Hiccup had followed him so when the dragons smell him they all try to attack him. He is a human and they have never really had to deal with that because they have been isolated from humans. 

After Toothless saves Hiccup from the dragons, he takes him back to Berk. This is when Hiccup realizes that he cannot continue to keep Toothless in Berk. He needs to be with his mate in the Hidden World. Hiccup makes the final decision to let Toothless go, and that results in all of the Vikings letting their dragons free. They all say their final goodbyes and watch them all fly away to their new home. After that, a time skip of 10 years happens in the movie. All of the Vikings are surviving perfectly well without the dragons and Hiccup has 2 kids. He ends up sailing towards the Hidden World to maybe catch a glimpse of Toothless. He is surprised to see that Toothless has 3 kids of his own. The movie ends with Hiccup riding Toothless again with his kids.