-->"I don't need luck though, I don't want it. I've always had to struggle and fight and that's made me strong. It's made me who I am."

With his body kidnapped by Zuko, and his spirit adrift in the Spirit World, Aang tries to find the Ocean and Moon spirits to ask for guidance and protection. Instead he finds himself facing the mysterious Face Stealer Koh, the only one with information on the spirits' true forms and location. Aang is able to return from the Spirit World, and his soul is sucked back towards his body, forming a streak of light which Sokka, Katara and Yue (riding Appa) use to track him. They defeat Zuko, but Aang insists on taking him captive instead of leaving him to die.

When they return, however, the Fire Nation have penetrated the walls of the city, and Zhao's mad plan is revealed. He has reached the Spirit Oasis, and despite Iroh's warning that trifling with the natural order of things could have [[ApocalypseHow serious consequences]], for all people and not just the Water Tribes, Zhao kills the Moon Spirit, causing a lunar eclipse and rendering Waterbenders powerless. Iroh, furious, [[BewareTheNiceOnes attacks Zhao]], who flees, Zuko in hot pursuit. Just when everyone thinks all hope is lost, Aang's Avatar State activates. Synchronizing with the Ocean Spirit, the two merge together, forming a huge anthropomorphization of water in the form of a skyscraper-sized, glowing koi fish leviathan ([[FanNickname "Koizilla"]]), which clears the Fire Nation attackers from the city in mere seconds, while the water tribe members show their respect by bowing out of reverence, also ensuring it doesn't sweep them away, too. Then, still unsatisfied, the spirit flattens the Fire Nation fleet with terrifying ease, literally shoving ships miles out into the arctic as if they were mere bath toys.

Iroh, trying to salvage things at the Spirit Oasis, notes Yue's white hair. Yue reveals that she was a stillbirth, but when her parents plunged her into the waters of the oasis, the Moon Spirit gave her part of its spark of life, which resulted in her whitewashed hair. She decides now to give that spark back, despite Sokka's protests. She bids him farewell as she closes her eyes and dies in his arms, reviving the slain white fish that is the Moon Spirit while her body dissolves into moonlight. After her physical body vanishes, however, Yue briefly reappears, now as the Moon Spirit, taking on a much more beautiful appearance. She says a final goodbye to Sokka, reassuring her love that she will always be with him, and the two embrace in a tender kiss... and Yue vanishes altogether. As the moon appears in the sky, La and Aang notice all is well once again and make their way back to the oasis. However, when Zhao spies the sudden return of the moon in a brief pause between his duel with Zuko, he's absolutely mortified. Having lost hope, he still tries to attack Zuko, only to be grabbed by a spiteful La still in Koizilla form, about to drag him to the depths of the water and drown him. Zuko reaches out to offer to save his life, but Zhao's pride is too resilient for him to show any sign of humilation, especially to his sworn personal enemy - so he opts to choose death instead. He surrenders to his fate, disdaining Zuko's offer of assistance. He disappears into the water, never to be seen again.

As the Gaang reunite and discuss their upcoming plans for the following spring, Sokka is left with a more heartbreaking task - breaking the grievous news to Chief Arnook that her daughter sacrificed herself to save Tui. Arnook knew all along this was bound to happen one day, as he once saw a prophetic dream foretelling Yue's transformation into a beautiful princess. He's proud of Yue, but deeply saddened that she's passed on to a higher existence. At the same time, Master Pakku and quite a few others, be it shellshocked citizens or generous souls, resolve to leave the chaos that just shook up their home and head down to the Southern Water Tribe to help rebuild, lending a hand to a fellow tribe in need.

However, far off in the heart of the Fire Nation, the survivors of the siege have taken refuge in their homeland to regroup and inform the Fire Lord about the catastrophe that befell their attack. Unfortunately, word has gotten out about Zhao's demise and Iroh's insubordination and subsequent assault toward a superior-ranked officer. The Fire Lord rants and raves about Zhao's incompetence, deems Iroh a traitor, and Zuko a failure out of impatience for failing to show up at his throne chamber with the Avatar already and keep this mess from even unfolding in the first place. Instead, he chooses to place his faith in a more trustworthy individual in capturing the Avatar, and gives unspecified commands to an [[TheDragon ever-obedient and just as ruthless girl]]: [[HeroKiller his daughter]].

* AscendToAHigherPlaneOfExistence: Yue's connection with the moon allows her to become the moon spirit when the original dies.
* AwakeningTheSleepingGiant: The Ocean and Moon Spirits seemed content to simply swim around in their pond and not actively assist the Water Tribe or oppose the Fire Nation. Then Zhao kills the Moon Spirit. The Ocean proceeds to merge with Aang and go on a RoaringRampageOfRevenge against the Fire Nation.
* BadassBoast: What Zhao does to the Moon spirit, Iroh will return tenfold. He doesn't succeed... but only because Zhao ran.
* BerserkButton: Harm to the spirits is one for Iroh.
** Swallowing his pride and accepting his mistakes is a big, fatal one for Zhao - twice. When told not to kill Tui, he lets his anger take control and kills the fish anyway. Later, when he's on the brink of death, his only means of survival is to grasp Zuko's hand - the hand of the kid he hates to the core. Instead of sucking it up, he lets his pride call the shots and lead him straight to a frigid, cold, watery grave.
* BittersweetEnding: The Fire Nation has been repelled and the day is won, but at the cost of Yue's life to Sokka's dismay. Meanwhile, Zuko and Iroh are unaware they've been branded traitors by their nation and a [[HeroKiller much worse threat]] than Zhao is on the horizon.
* BloodlessCarnage: The invasion itself. After Koizilla wipes out the fleet, we never see any dead bodies; considering Zhao's fate, that hints at a rather horrifying possibility for why they're all missing.
** More remarkable is how the enraged Water Spirit is capable of wiping out ''only'' the Fire Nation's troops: the nearby Water tribesmen who bowed down in supplication were spared. We actually see the water pass over them harmlessly.
* CallBack: Hei Bai, the forest spirit in "Winter Solstice: Part 1", returns to aid Aang in the Spirit World.
* CasualDangerDialogue: While Iroh and Zhao are discussing their strategy on their ship, Hahn interrupts, charging at Zhao, who ''casually tosses him overboard'' and resumes the conversation with, "As I was saying..."
** A more frightening variant: Aang talks flatly and casually in front of Koh the entire time he's speaking to him - should he show even a speck of emotion, Koh would steal his face. When Aang suddenly has an epiphany about the koi fish behind Koh's back, it's enough of a realization to make him drop his facade and get a cheerful reaction out of him - and Koh wheels around at the speed of light ready to pounce at Aang's slip-up... only to find Aang has regathered himself and gone dull and emotionless again. To top it all off, Koh taunts him out of spite as he walks away.
* ChekhovsGunman: That girl who appeared in the flashback in "[[Recap/AvatarTheLastAirbenderTheStorm The Storm]]" as well in the opening turns out to be the Firelord's daughter and Zuko's sister. At the end, the firelord sends her to replace Zhao. We still have yet to learn her name, though.
* CityOfCanals: The Northern Water Tribe's city.
* ClaimedByTheSupernatural: Yue's white hair can be a considered a benign example.
* CurbStompBattle: Aang combines with the Ocean Spirit and utterly destroys the Fire Nation fleet. The same goes for Zhao and the Moon Spirit - he's doomed the moment it picks him up in its clutches.
** Also Katara's rematch against Zuko, though she even points out the [[WorfHadtheFlu inequality of the matchup]].
* DeliberatelyMonochrome: While Zhao has the Moon Spirit captive and after he kills it. During the Moonless sequence, the only color comes from firebending and Yue's irises -- in other words, supernatural sources. This tips off Iroh to Yue's true nature.
* DidntThinkThisThrough: Two examples: Zuko's plan was, apparently, "Grab the Avatar and run off into the frozen tundra" (Iroh later calls him on it). Zhao, meanwhile, seems never to have considered any possible consequences to ''destroying the Moon'' apart from "no water bending."
* DoNotTauntCthulhu: Iroh warns Zhao about the disastrous consequences of messing with the spirits. The Ocean Spirit's wrath when Zhao kills the Moon Spirit proves him correct.
* DreamingOfThingsToCome: Yue's father dreamed about her becoming the Moon spirit and knew that day would come.
* DrivenToSuicide: Zhao commits suicide by letting the ocean spirit take him after failing to defeat Zuko.
* EveryoneHasStandards: Iroh makes it clear to Zhao that he doesn't believe in killing a spirit to ensure victory for the Fire Nation. Just when Zhao's about to kill it, Iroh sides with Aang on his claim that everyone needs the Moon.
* FaceStealer: Trope Namer. And my, is Koh creepy.
* AGodAmI: Zhao does this after capturing the Moon Spirit.
* GodInHumanForm: Zhao learned from a special library that the black and white koi fish in the oasis are actually the Moon and Ocean Spirits. He chose to use this knowledge to destroy the moon, which he believed would cripple the Water Tribe only.
* GodzillaThreshold: The defeat of Zhao's fleet is this to the Fire Nation, as the Fire Lord sends his daughter to drop the hammer.
* HeroicSacrifice: Yue makes one to save the Moon Spirit and her tribe.
* {{Jerkass}}: The Strange Spirit first encounters in the Spirit World. Rather than help Aang, he tries to shoo him away so that he can meditate in peace. He even voices his relief once Aang leaves, which led to Hei Bai getting angry with him.
* KarmicDeath: Zhao is killed by the Ocean Spirit for (temporarily) slaying the Moon Spirit. Double whammy when you factor in he had a chance to be saved by Zuko and chose to die at La's hands, and so he was subjected to self-inflicted karmic death.
* MeaningfulName: 'Yue' is Chinese for 'moon', which fits how the moon spirit gave energy to save her life as a baby. It becomes all the more fitting when she literally becomes the moon.
* NonchalantDodge: Again, Zhao avoiding Hahn's assassination attempt.
* OffhandBackhand: A near-complete reversal of the trope, as it is done by the villain on [[TooDumbtoLive someone who didn't even stay quiet]] [[WhatanIdiot while sneaking up.]]
* OhCrap: The appearance of Zuko's sister is a clear sign that Team Avatar is in for trouble, BIGGER trouble.
* OneWingedAngel: [[FanNickname Koizilla]]
** A (more) heroic example: Princess Yue becoming the Moon Spirit.
* PyrrhicVictory: After the battle, half the Fire Nation fleet has been decimated and Zhao has been killed, however, the Northern Water Tribe infrastructure has been heavily damaged, they've exhausted almost every resource during the battle such that it would take months to recover and Princess Yue died to revive the Moon Spirit. Pyrrhic indeed.
* RedSkyTakeWarning: While Zhao has the Moon Spirit captive, the moon's light turns red. It's definitely a bad omen.
* TheReveal: That girl from the flashback in "The Storm" who smirked devilishly at Zuko's scarring is none other than his sister. She was smiling out of cruel satisfaction.
* RiddleForTheAges: As far as we know that [[WesternAnimation/TheLegendOfKorra the next Avatar]] is related to the then-current Chief, who would be Chief Arnook's successor since his only child became the Moon?
* RoaringRampageOfRevenge: The Ocean Spirit fuses with Aang and lashes out at the Fire Nation after the Moon Spirit is killed. It does calm down after Yue gives her life to the moon.
* SeasonFinale
* SaveTheVillain: To Sokka's annoyance, Aang won't let Zuko die in the cold. Zuko later attempts to save Zhao from the Ocean Spirit but fails.
* SplashOfColor: From firebending and Yue's irises after the switch to DeliberatelyMonochrome.
* TakeMyHand: Zuko yells this when the Ocean Spirit grabs Zhao. Zhao refuses his enemy's help and lets the spirit drag him underwater.
* TooDumbToLive: Hahn's 'ambush' on Zhao involves getting onto the ship and charging him, loudly declaring his intent to kill him as he runs forward, weapon ready to strike, when his opponent is a powerful bender, has ''another'' powerful bender next to him, and is currently on a ship full of soldiers. Obviously, it doesn't work.
* WhamEpisode: Admiral Zhao dies, Katara is officially a Waterbending master, and the last two minutes of the episode introduce a new and deadly playmate for the crew next season... one Zuko's ''sister''.
* WhamShot: Why hello there, '''Zuko's Sister'''.
* WhatHappenedToTheMouse: Hahn's fate after being chucked overboard Zhao's ship is left uncertain. For all we know, he's an ice cube chilling at the bottom of the ocean by now. But after being such a {{Jerkass}} as he is, [[KickTheSonOfABitch nobody really gives a rip]].
** Koh mentions that he and Aang will meet again but never come back in the series. It actually happens in the online game ''Escape from the Spirit World''.
* WillOTheWisp: After being transported to the spirit world, Aang runs through a swampy region chasing a mysterious small light sphere. When he climbed up a tree and grabbed it, it made the tree branch dissolve.