How To Train Your Dragon 2 is the sequal (released in June 13th, 2014) to How to Train your Dragon, which was released in 2010. A third and final chapter is currently under way, and is due to be released June 29th, 2018.
About (contains spoilers!)
While Hiccup and Toothless are out exploring the world (five years later later after the first film), Hiccup discovers another piece of land when he and Toothless nearly crash after flying solo together. Hiccup names the new land "Itchy Armpit," and adds it to a ever expanding map of the world he's working on. Astrid, who's Hiccups love interest, flies in on Stormfly and notices a strange ice formation off in the distance. They head out to take a closer look and find that a dragon trapping fort had been blown to bits. Eret, who works for Drago Bludvist, confronts them and accuses them of destroying their fort.
Back on Berk, Hiccup is trying to explain to his dad, the chief who's getting ready to pass on his roll to Hiccup, about the new land and the hostile trappers they've discovered. Hiccup finally gets his dad to listen after he tries to pronounce "Drago Bludvist" and mentions that he's creating a dragon army. Concerned, Stoick then orders everyone to make their way to the underground stables with themselves and their dragons to avoid a war. Hiccup refuses to stay locked up and is convinced that he can reason with Drago and sets off on Toothless with Astrid behind him.
Out on sea, Eret and his crew are making their way back to Drago's base empty-handed. However, Hiccup and Astrid land on their ship, pretending to surrender themselves and their dragons to trick them into bringing them to Drago. However, moments later, Snotlout, the twins, Fishlegs, Gobber and Stoick land on Eret's ship in attempt to rescue Hiccup and Astrid. But as stubborn as Hiccup can be, he refuses to go home and rides off on Toothless alone.
As Toothless and Hiccup are flying over the open ocean, a mysterious figure emerges from the clouds, riding on a owl-like dragon. The mysterious figure snatches up Hiccup, leaving Toothless alone in the cold and icy water (he's rescued by another dragon and reunited with Hiccup later on). Hiccup is taken to a dark and strange place where he's trying to protect himself from other dragons. Amazed by his skills with dragons, the figure approaches Hiccup and realizes that the person she has captured is her son (a scar on his chin identifies him).
Hiccup and Toothless are then taken into her dragon sanctuary (AKA: The home of the Great Bewilderbeast) where he learns that she's been rescuing dragons for the past twenty years. Hiccup then mentions that Berk has changed for the better, but his mother doesn't believe him. She tells Hiccup the story about how Cloudjumper, her dragon, broke into their home, finding him in the cradle. She rushed to protect Hiccup, but what she saw was proof in everything she believed in. Dragons weren't viscous beasts, but intelligent and gentle creatures. However, Stoick comes in and attacks Cloudjumper, causing him to set their home ablaze. Cloudjumper then takes Valka and flies away.
Assuming that Stoick and Gobber was captured by Drago after they set out once more to search for Hiccup, Astrid, Snotlout, the twins and Fishlegs find Eret's ship and kidnap him. Eret refuses to lead them to Drago at first but gives in.
Back at the dragon sanctuary, Hiccup and Toothless are preparing to head out to find Drago once more. But as they were doing so, Stoick and Gobber find the sanctuary and make a second attempt to bring Hiccup home. While making their way out of the sanctuary, Hiccup is trying to explain to his dad that he has found his mother. Upon reaching the exit, Gobber turns around and tells Stoick that he should take control of the situation ahead. He pulls out his sword, walks into a room of ice, and finds his long lost wife.
Outside of the sanctuary, Drago is preparing to attack and take down their alpha. During the battle, Hiccup confronts Drago and tries to show him how dragons are loyal creatures. Drago refuses to listen to Hiccup and says "no... let me show you." He commands his alpha to take control of Toothless's mind. He turns against Hiccup and starts backing him into a corner, getting ready to strike. Stoick notices and rushes over to save Hiccup. As he jumps in front of Toothless to push Hiccup out of the way, Toothless fires his plasma blast, killing Stoick.
After saying their goodbyes to Stoick, Hiccup comes into his own and leads the others back to Berk to take on Drago once more. When they arrive Hiccup makes an attempt to bring Toothless out of the trance the Alpha has him in. He sucseeds and hops onto his back and goes on his way to take down Drago. After being frozen in a mound of ice, Toothless is able to break free from the ice and challenges the alpha. He defeats the alpha, and the alpha dives into the water, taking Drago with him.
After Toothless is saluted as the new alpha, and the inhabitants of Berk are reunited with their dragons, Hiccup is announced as the village's new chief.