It starts when hiccup found toothless and his dad yelled at him he leaves Berk and becomes the lone dragon rider and saves each dragon from being killed and everyone on Berk knows about the lone dragon rider but they don't know who he is so they try to capture hiccup and they fail because hiccup has learned to be very skilled with not being seen nor caught by others hiccup lives on an island by himself far away from Berk so it's hard to send people out there to find them Stoick thinks his son is dead he becomes more stern and yells more at others
Astrid soon figures out who the lone dragon rider is and tries to confront hiccup but fails after he escapes on toothless and flies off back to his island there he designs weapons, and dragon based stuff and studies every dragon and rescues each dragon he saves and puts them in his island where they can live peacefully without harm from others and they are very fond of hiccup for saving them and toothless becomes the "king" of the island where the dragons live
Years pass and everyone forgets about the lone dragon rider even Stoick
Hiccup still lives on the island and doesn't remember Berk and soon he meets his mom and they both become dragon riders that save dragons from harm