The Season Finale. Zuko has taken Aang's body, but now struggles to find shelter in the freezing temperatures of the North Pole, while Sokka, Katara and Yue search for him and Aang on Appa. Meanwhile, Aang's spirit, still in the Spirit World, goes to the menacing Koh the Face Stealer for information on the locations of the Moon and Ocean Spirits.
At the Northern Water Tribe, Firebenders and Fire Nation tanks have breached the outer wall, and are now assaulting the smaller inner wall. Iroh cautions that they're still on a deadline — once the full moon rises, the Waterbenders will be unstoppable. And that's when Admiral Zhao reveals his master stroke — take away the Waterbender's abilities by slaying the moon spirit. Iroh is horrified by the plan, but Zhao rebuffs him.
Cut back to Zuko and Aang. Zuko starts reminiscing to Aang's unconscious body how there's always something getting in the way, like the blizzard currently keeping them trapped. He says Aang would never understand this — that he's like Zuko's little sister, a Firebending prodigy who's always been Born Lucky, while he was lucky to be born.
Meanwhile, the Fire Nation soldiers have broken through the second wall and are now well into the city, but night has fallen, and the Waterbenders, newly empowered by the full moon, are easily beating them back. Aang learns from Koh that far from needing the spirit's help, it's the other way around — if he doesn't do something, someone's going to kill them. Thanks to Koh's hints, Aang works out that the Moon and the Ocean spirits are actually the koi fish from the last episode, and some help from Hei-Bai quickly returns him to his body — and just in time too, for the flash of Aang's spirit returning to the physical world has clued in the nearby rescue party on Aang's whereabouts. Katara defeats Zuko easily, but Aang refuses to leave the exiled prince to die in the snow, taking him with them.
Zhao shows up in the hidden oasis to grab the moon spirit and stuff it into a bag, causing the moon to turn blood red and shutting off all Waterbender's abilities. The spiritual disturbance is felt by both Aang and Yue, who reveals that the Moon Spirit saved her life as a child by putting a bit of its essence into her.
Aang shows up just in time to see Zhao about to finish off the Moon Spirit. Aang pleads with the power-hungry Admiral that hurting the Moon Spirit will hurt the entire world in ways he can't even imagine. Iroh chimes in, threatening Zhao that he'll do tenfold whatever Zhao does to that spirit. Seemingly cowed, Zhao puts the spirit back in the oasis — before killing it with a fire blast!
The moon goes dark. Iroh unleashes a Firebending assault of such fury that Zhao immediately flees — only to run into an escaped Zuko, who is furious that Zhao tried to assassinate him. Aang goes into the Avatar State and, in joining with the Ocean Spirit, destroys the entire Fire Nation armada and army. Yue realizes that she can save the Moon Spirit by returning the life it gave her. She sacrifices herself to become the new Moon Spirit, sharing one last kiss with a grief-stricken Sokka as she fades away.
The arrival of the moon in the sky causes the Ocean Spirit to stop its rampage, leaving Aang on the outer walls of the city. Zhao is, in short order, defeated by Zuko, shocked when he sees the moon back in the sky, and pulled underwater by the Ocean Spirit in retaliation for slaying the Moon. Despite everything Zhao has done to him, Zuko still tries to rescue Zhao — but the Admiral refuses to accept Zuko's help, allowing the spirit to drag him under the waves. Zhao is never seen again — in this series, at least.
Dawn breaks on a battered but still standing Northern Water Tribe. Master Pakku announces he and a few other Waterbenders are going to the South Pole to rebuild their sister tribe. Katara's glad to hear this, but is concerned about Aang, who still has to master waterbending. Pakku says he'd better get used to calling her "master," then. Sokka shares a moment with Chief Arnook, who reveals he once received a vision of his daughter becoming the moon, and knew this day would come. And so, with heavy hearts but high hopes, Aang, Sokka, and Katata prepare for the next leg of their journey.
Meanwhile, in the Fire Nation, Fire Lord Ozai is talking to his daughter. He says Iroh is a traitor, her brother Zuko is a failure, and he has a task for her...
- Ascend to a Higher Plane of Existence: Yue's connection with the moon allows her to become the Moon Spirit when the original dies.
- Awakening the Sleeping Giant: 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. Iroh warns Zhao about the disastrous consequences of messing with the spirits but Zhao kills the Moon Spirit. The Ocean proceeds to merge with Aang and go on a Roaring Rampage of Revenge against the Fire Nation.
- Badass Boast: What Zhao does to the Moon spirit, Iroh will return tenfold. He doesn't succeed... but only because Zhao ran.
- Katara also gives one just before she curb-stomps Zuko.Katara: Trust me, Zuko. It's not gonna be much of a match.
- Katara also gives one just before she curb-stomps Zuko.
- Berserk Button: Harm to the spirits is one for Iroh.
- Be Careful What You Wish For: Prior to going to find the Moon and Ocean Spirits, Aang wonders if they're going to give him some super spirit power that would crush the Fire Nation. La, the Ocean Spirit, does just that... but given the circumstances, Aang's not happy.
- Beware the Nice Ones: Once Zhao kills the Moon Spirit, Iroh and Aang get to UNLEASH in a way we havent yet seen in the series. Up to Eleven with Koizilla.
- Bittersweet Ending: 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 much worse threat than Zhao is on the horizon.
- Bloodless Carnage: 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 Ocean Spirit is capable of wiping out only the Fire Nation's troops: the nearby Water tribesmen who bowed down in supplication were spared.
- Book-Ends: In the first episode of the season, Iroh tells Zuko "A man needs his rest" when trying to convince Zuko to just go to bed. Zuko just brushes him off. At the end of this episode, Iroh says it again after Zuko admits to being too tired to chase the Avatar. This time Zuko listens and decides to lie down.
- Broke Your Arm Punching Out Cthulhu: Zhao succeeds in killing the Moon Spirit... but his entire fleet is destroyed by the enraged Ocean Spirit, and he himself is dragged to an uncertain but probably terrible fate.
- Call-Back: Hei Bai, the forest spirit in "Winter Solstice: Part 1", returns to aid Aang in the Spirit World.
- Casual Danger Dialogue:
- 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.
- Chekhov's Gunman: That girl who appeared in the flashback in "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 don't learn her name until the next episode, however.
- City of Canals: The Northern Water Tribe's city.
- Claimed by the Supernatural: Yue's Mystical White Hair can be a considered a benign example.
- Curb-Stomp Battle:
- Aang combines with the Ocean Spirit and utterly destroys the Fire Nation fleet.
- Also Katara's rematch against Zuko, though she even points out the inequality of the matchup.
- Zhao barely has to pause his conversation to deal with Hahn.
- Deliberately Monochrome: 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.
- Didn't Think This Through: 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', even after Iroh warns him that messing with the spirits is a bad idea.
- The latter is even more egregious than it looks when considered on a physical, instead of spiritual, level: the Moon controls the tides, and the Fire Nation is an archipelago.
- Dragged Off to Hell: Zhaos ultimate fate. Viewers would have to wait until The Legend of Korra to find out what exactly happened to him.
- Dreaming of Things to Come: Yue's father dreamed about her becoming the Moon spirit and knew that day would come.
- Dressing as the Enemy: Hahn and his men disguise as Firebender mooks in order to get close to Zhao. It helps that the outfit includes face-obscuring helmets.
- Driven to Suicide: Zhao lets the Ocean Spirit take him after failing to defeat Zuko.
- Everyone Has Standards: 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.
- Face Stealer: Trope Namer. And boy, is Koh creepy.
- Fatal Flaw: His inability to swallow his pride and accept his mistakes is a big 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 being dragged away by La, his only means of escape 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 Fate Worse than Death.
- Foreshadowing: Zhao tells Iroh all about the time he discovered an underground library that contained information on the spirit world.
- Gaia's Vengeance: Koizilla sparing the reverent Waterbenders and unleashing the wrath of nature unto the Fire Nation is not unlike the Judgement of God in Genesis, Revelation or elsewhere in the Bible.
- A God Am I: Zhao does this after capturing the Moon Spirit. Complete with a Milking the Giant Cow gesture.
- God in Human Form: 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 was his destiny.
- Good Is Not Soft: This is the episode where Uncle Iroh makes his HeelFace Turn. Even before this point, he's always been an Anti-Villain who is affable and kind to everyone he isn't fighting at the moment. That doesn't mean he's going to make any effort to rescue a Water Tribe assassin who just tried to kill one of his colleagues right in front of him.
- Greater-Scope Villain: Azula is teased as this in The Stinger.
- HeelFace Turn: Iroh stands with the Gaang against Zhao, and is branded a traitor to the Fire Nation. Downplayed as he was always a non-malicious Token Good Teammate and Punch-Clock Villain.
- Heroic Sacrifice: Yue makes one to save the Moon Spirit and her tribe.
- He's Dead, Jim: When Yue collapses into Sokka's arms, the latter diagnoses her dead after a second of feeling her pulse.
- Jerkass: The strange Spirit Aang 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.
- Karmic Death: Zhao doesn't actually die, but he is dragged off to an uncertain doom 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 refused it.
- Kubrick Stare: Zuko's sister does one right at the camera as the final shot of the season.
- Last Kiss: Between Sokka and Yue before she Ascends to a Higher Plane of Existence.
- Meaningful Name: 'Yue' is Chinese for 'moon'; her mother named Yue after the Moon Spirit gave energy to save her life as a baby. It becomes all the more fitting when she literally becomes the moon.
- No Body Left Behind: Yue's body dissolves in Sokka's arms.
- Nonchalant Dodge: Zhao avoiding Hahn's assassination attempt.
- Oh, Crap!:
- The appearance of Zuko's sister is a clear sign that Team Avatar is in for trouble, BIG trouble.
- When Iroh starts to make good on his Badass Boast, Zhao is visibly terrified and makes a run for it.
- One-Winged Angel:
- Koizilla, the result of Aang in the Avatar State channeling the Ocean Spirit.
- A (more) heroic example: Princess Yue becoming the Moon Spirit.
- Pose of Supplication: Members of the Waterbender tribe bow down to Koizilla and are spared while the Firebender troops get washed away.
- Red Sky, Take Warning: While Zhao has the Moon Spirit captive, the moon's light turns red. It's definitely a bad omen.
- The Reveal: 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.
- Riddle for the Ages: As far as we know that the next Avatar is related to the then-current Chief, who would be Chief Arnook's successor since his only child became the Moon?
- Roaring Rampage of Revenge: 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.
- Save the Villain: 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.
- Seen It All: A Water Tribe infiltrator in obsolete armor rips his helmet off, screams a threat at Zhao, charges him with a spear, and gets thrown overboard with so little effort that Zhao barely pauses in speaking. Iroh just shakes his head. He's seen some inept assassination attempts before, but this one just takes the cake.
- Shout-Out: That monkey spirit Aang meets while in the spirit world — he is seen meditating in a yoga position of some sort, and when he addresses Aang, he speaks in a clipped accent. Sound familiar?
- Splash of Color: From firebending and Yue's irises after the switch to Deliberately Monochrome.
- Take My Hand: Zuko yells this when the Ocean Spirit grabs Zhao. Zhao refuses his enemy's help and lets the spirit drag him underwater.
- Too Dumb to Live: 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.
- Voice of the Legion: The second time we hear Aangs Avatar voice, with twice as many words as last time. And twice the bone-chilling factor.
- Wham Episode: 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.
- Wham Line: "Iroh is a traitor, and your brother Zuko is a failure".
- Wham Shot: Why hello there, Zuko's Sister.
- What Happened to the Mouse?:
- 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, nobody really gives a rip.
- Koh mentions that he and Aang will meet again but never comes back in the series. It actually happens in the online game Escape from the Spirit World.
- Will-o'-the-Wisp: 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.