I do believe that there's more plot progression in this single book than in all of the previous books combined. Wow. Just...wow.
This book tells the story of Hiccup's ancestor, Grimbeard the Ghastly (the one whose treasure was the main plot point of the second book in the series), uncovers Grimbeard's throne, which is heavily tied to the prophecy mentioned by Norbert the Nutjob, tells the story of Hiccup the First and Second, explains what the name Hiccup means, explains about Hiccup's family tree, goes into more detail about the divergence in Fate when Humongously Hotshot failed in his quest and Valhallarama marries Stoick the Vast instead, gives us the prophecy in its entirety, sets in motion the great war between humans and dragons that will end in the extinction of dragons, and, just in case that wasn't enough plot for you, the book also decides to delve into the mystery of Fishleg's paternity and lobster pots turned into rucksacks.
Yeah. A lot happened.
And, of course, this marks the end of the books I've actually managed to get a hold of so far. So now I've got to wait to find out what happens.
On the plus side, I totally pictured Alvin the Treacherous like this the entire time: