"Avatar: The Last Airbender" ended with peace being reestablished between the four elemental nations -- but there were still some dangling threads. "The Promise Part 1 (Avatar: The Last Airbender Book Four)" picks up where the series left off, and addresses the problems of restoring the Fire Nation to how it should be -- especially when not everyone agrees what that is. 
 Now that there is peace between the Fire Nation and Earth Kingdom, Zuko implements the "Harmony Restoration Movement" (named by Sokka) -- the Fire Nation will relocate colonists who have been living in the Earth kingdom for the past century. And while everyone is celebrating, Zuko also extracts a promise from Aang: if he ever becomes like his father, he wants Aang to kill him. 
 But after an assassination attempt, Zuko unexpectedly withdraws the Harmony Restoration Movement. It turns out to be a little more complicated than Zuko expected -- the Fire Nation citizens there have intermarried with the Earth Kingdom people, and don't want to be uprooted from their homes. Now not only must Aang and Zuko deal with colonials who don't want to leave, but also with "freedom fighters" who want the Fire Nation gone. 
 "The Promise Part 1 (Avatar: The Last Airbender Book Four)" is a story that I would love to see translated into an animated movie -- there's a feeling of warm, comfy familiarity in seeing all our old characters reunited again. It springs pretty seamlessly from the end of the TV series, showing where the characters have gone since and the troubles of a post-war world. 
 The primary problem is the story is very short, so sometimes it feels like the characters are flipping between emotions too fast -- Aang literally goes from zero to Avatar State and almost kills Zuko. Too fast! 
 However, it's otherwise a very solid story, with fire/ice battles and some fun comic relief from Sokka (the "oogies" he gets whenever he sees Aang and Katara kissing). And it revolves around a situation with no easy answers. Aang believes that the four nations must be distinct and some Earth people are violently opposed to the Fire Nation, but Zuko doesn't want to wreck the lives of the people who have become rooted in the Earth Kingdom. There's no "win" scenario. 
 Despite being the Fire Lord, Zuko is still morose and introverted -- partly because of his worries about becoming like his father, and partly because of his inexperience at ruling an entire nation. He simply doesn't know what to do, and it's eating him up. For some reason he doesn't go to Iroh for advice even though the old guy has never let him down before. 
 We also get some developments for other characters -- Aang and Katara are enjoying their new romance, although the "sweetie" thing is a bit cloying. But they get some nice moments here (Katara commanding a guard to not set her boyfriend on fire), and Toph is a pretty hard-nosed metalbending teacher. 
 "The Promise Part 1 (Avatar: The Last Airbender Book Four)" ends on a slight cliffhanger, leaving us wondering what will happen next for Zuko and Aang. It's going to be a long wait for the next part.