A year after Hiccup united the human and dragon world, he all of a sudden, leaves. The whole village is wondering where he and Toothless are. Since the boy she was in love with left without saying anything, Astrid goes into a secret stage of depression.
But the week after Hiccup left, Astrid receives a letter. And after that day, Astrid has gotten one letter each week for five years. But one week, Astrid doesn't get a letter, neither does she the next.
Astrid decides to find out who had written all of these letters for her, and on the road she gets clues to who this mysterious man is.