When Hiccup was just a young boy, he was isolated for being the runt. So he created an imaginary friend, named Jack Frost. Every winter, Jack would come back to Hiccup and the two would have the best nights of their life. But when Hiccup was 15, Jack didn't come back. He waited years and years but he didn't return. When Hiccup was 18 and soon to be married to Astrid, Jack came home. And turned Hiccups' life upside down.