-->"It's the Fire Nation. They've closed in on the North Pole."

In the Northern Water Tribe, things are going great for our heroes. Aang and Katara are advancing quickly in their water bending lessons, and Sokka's friendship with Yue grows, even though he knows he can't have more. That all changes when they discover the Fire Nation Navy advancing with hundreds of ships.

With no other choice, the Water Tribe is forced to go to war; Aang attempts to attack the ships head on, while Sokka volunteers for a guerrilla mission, facing both death and Yue's jerk fiance Hahn. In the Fire Nation camp, Zuko hides in a disguise, before going solo into the Water Tribe in an attempt to kidnap Aang. Zhao, meanwhile, alludes to a secret plan of his own: to kill the Moon Spirit, the source of all Waterbending.

The reason Zhao can do this at the North Pole is that here, in the Spirit Oasis, the Northern Water Tribe's inner sanctum, live Tui and La, the spirits of the Moon and Ocean respectively. They inhabit the bodies of two koi, circling endlessly in the oasis' pool. Aang, his battle against the fleet fruitless, comes here to meditate, hoping to bring the spirits in on his side; Katara stays to defend him while Sokka and Yue oversee the defenses. It is here that Zuko finds him, and though Katara [[TookALevelInBadass puts up one hell of a defense]], Zuko defeats her and kidnaps Aang. This has the side effect of stranding Aang in the Spirit World, as his body is not where he left it. The episode ends with the walls of the city breached and Zuko toting Aang across the frozen tundra.

* AccidentalMisnaming: Hahn can't get anyone's names right.
* AlertnessBlink: When Aang is destroying the Fire Nation ship's catapults, he is surprised by a soldier, causing two white flashes next to him and a clear blinking sound.
* AnswerCut: When Arnook says "It is with great sadness I call my family here before me, knowing well that some of these faces are about to vanish from our tribe," the camera flashes to Yue, Pakku, and Hahn.
* BadassBoast: See TakeThat and keep in mind that this show is the only show to beat out ''WesternAnimation/SpongebobSquarepants'' at the Kids Choice Awards.
* CallBack: Some of the waterbenders stop a Fire Nation ship like the one seen in "[[Recap/AvatarTheLastAirbenderTheBoyInTheIceberg The Boy in the Iceberg]]".
* CityOfCanals: The Northern Water Tribe's city.
* CurbStompBattle: At night under the full moon, Katara thumps Zuko repeatedly with her improved training and special tide-induced boosts. But then the sun rises, and Zuko's power boosts activate, letting him burn through all ice attacks and knock Katara out with one blow. They have a ''third'' battle later after finding Aang, where Katara curb stomps again.
* DualWielding: While sabotaging the Fire Nation ships, Aang gets attacked by a soldier wielding two hammers attached by chains.
* FlightOfRomance: Sokka takes Yue on one. The Fire Nation ruins it.
* {{Foreshadowing}}: Zhao tells Iroh all about the time he discovered an [[Recap/AvatarTheLastAirbenderTheLibrary underground library]]
* InstantArmor: Aang does this with snow as a joke.
* JawDrop: Sokka's reaction when he learns that Hahn is Yue's fiancé.
* {{Jerkass}}: Hahn. He tells right to Sokka's face that's pursuing Yue merely for the perks of being the chieftain's son-in-law.
* 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.
* TheNativeRival: Hahn, to Sokka.
* NoisyNature: Zuko finds some sealion-turtles that won't stop making noise while infiltrating the North Pole, and yells at them to be quiet.
* NonindicativeName: Not technically a siege.
* OhCrap: Aang's reaction when, after subduing the first Fire Nation ship, he sees that there are still ''hundreds'' on their way.
* OneManArmy: Averted. Despite being possibly the most powerful bender in the world, Aang can't single-handedly take on a whole navy.
* OpeningShoutOut: When Aang decides to actively fight against the Fire Nation invasion, there is a shot of him from behind holding his staff, followed by a pan up into the sky, which is identical to that at the end of the opening sequence.
* PhlebotinumBattery[=/=]FieldPowerEffect: "You rise with the moon. I rise with the sun."
* TheSiege
* SnowMeansDeath: Black snow, to be precise.
* TakeThat: Confirmed by WordOfGod.
-->'''Pakku:''' A couple years and you might be ready to fight a [[WesternAnimation/SpongeBobSquarePants sea sponge]].
* TookALevelInBadass: Katara's fighting skill was already good enough to give Master Pakku a decent workout in the previous episode. Within a very short time, Katara's at the top of the class pummeling all the other trainees in under 30 seconds.
** Zuko even acknowledges it with a complimentary insult: "You little peasant, you found a master haven't you?"
* UnwantedAssistance:
--> '''Katara''': He's meditating. Trying to cross over into the Spirit World. It takes all his concentration.
--> '''Yue''': Is there any way we can help?
--> '''Aang''': How about some quiet?! Come on, guys, I can hear every word you're saying!
* UpperClassTwit: Hahn is essentially the Northern Water Tribe's equivalent. He even insults Sokka based on the fact that he's from the Southern Water Tribe, and states that he "wouldn't understand the political complexities of our tribe". Said political complexity is Hahn wanting to marry Yue for the perks. He also gets annoyed when Sokka points out the stolen Fire Nation armour is several decades out of date, and can't get Zhao's name right at all.
* WorfHadTheFlu: Played both ways; both times Katara beat Zuko she had the moon on her side and he had trudged through the arctic (which would weaken anyone but especially firebenders). The one time he beat her, he had the sun and also managed to get her from behind with a lucky shot.