In the Northern Water Tribe, things are going well for our heroes. Katara and Aang are still taking waterbending lessons from Master Pakku. Katara in particular has advanced quickly — faster than any student that Pakku has taught before — while Aang's tendency to goof off has made him fall behind. Meanwhile, Sokka's friendship with Yue grows, even though they both know that they can't have more.
But all that grinds to a halt when black snow starts raining down on the Northern Tribe— a result of the soot from hundreds of Fire Nation ships advancing to conquer and destroy the last bastion of the Water Tribes.
With no other choice, the Water Tribe goes to war; Aang, remembering how he wasn't there when the Fire Nation wiped out his own people, vows that he will make a difference this time. Yue tells Sokka that she can't see him anymore, as her feelings for him make things too confusing. Sokka deals with this by volunteering for a guerrilla mission, facing both death and, to his surprise, Yue's jerk fiance Hahn.
The first day of the invasion sees the Fire Nation Navy bombard the Northern Tribe with fireballs from a distance, while Aang and a few Waterbenders take on the ships head on. It goes badly for the defenders— there's just too many ships for them to handle. Meanwhile, Sokka is pulled off the top-secret mission after getting into a fight with Hahn, although the Water Chief later admits that he had an ulterior motive; he wants Sokka to guard his daughter.
As night falls, Iroh, acting as advisor to Zhao, recommends withdrawal, as the moon is nearly full, which empowers waterbending. Zhao agrees, while stating that he's working on a solution to "the moon problem."
During the cover of night, Zuko, who has been hiding among Zhao's crew in disguise all this time, sneaks into the Water Tribe in an attempt to kidnap Aang. Meanwhile, Aang, his battle against the fleet fruitless, gets an idea; the Water Tribe worships the Moon and Ocean spirits as the first Waterbenders, so how about he gets their help? Yue and Katara think that's a great idea, with one caveat; the last time Aang entered the Spirit World, it was totally by accident, so how's he supposed to do it on purpose? Yue helps by leading Aang and Katara to a secret oasis that's the spiritual center of the Northern Tribe, figuring that it'll make it easier for Aang to cross over. Aang meditates for a while, before crossing over as he watches a pair of black-and-white koi fish in the oasis that definitely won't be important later.
It is here that Zuko finds him, and though Katara puts up one hell of a defense, the rise of the sun gives Zuko enough of a boost to defeat her and kidnap Aang. This has the side effect of stranding Aang in the Spirit World, as his body is not where he left it. Furthermore, the rising of the sun means that the Fire Navy resumes its attack on the Water Tribe. The episode ends with the walls of the city breached and Zuko toting Aang across the frozen tundra.
- Accidental Misnaming: Hahn can't get anyone's names right.
- Alertness Blink: 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.
- Answer Cut: 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— one of whom will make it, one who does not, and one that we never find out about.
- Attack! Attack! Attack!: Averted, or perhaps subverted considering the character in question. Zhao sees the merit in Iroh's advice not to rush in and begin battle with the looming emergence of the moon, from which the waterbenders can draw greater power; he instead anchors his navy while ironing out a plan to deal with the problem.
- The Bad Guy Wins: Sort of. After being put through a lot of crap, Zuko finally captures the Avatar and escapes. Of course, now he has a blizzard to contend with...
- Badass Boast: See Take That!, and keep in mind that this show is the only show to beat out Spongebob Squarepants at the Kids Choice Awards.
- Big Damn Heroes: Once again, Appa saves the day.
- Braving the Blizzard: Zuko captures Aang (while his spirit is off in the Spirit World) and takes him further up north to avoid Zhao and his forces. It goes poorly because the North Pole is barren for miles and he ends up encountering a violent blizzard.
- Call-Back: Some of the waterbenders stop a Fire Nation ship like the one seen in "The Boy in the Iceberg".
- City of Canals: The Northern Water Tribe's city.
- Curb-Stomp Battle:
- The episode opens with Katara easily defeating another pupil named Sangok. Apparently he was simply the last of a line of other pupils.
- At night under the full moon, Katara thumps Zuko repeatedly with her improved training and special tide-induced boost. But then the sun rises, and Zuko's power boost activates, letting him melt through the ice that Katara had trapped him in and knock her out in a surprise attack.
- Didn't Think This Through: The Northern Water Tribe has been in near total isolation for almost a century, so when they pull out "real" Fire Navy uniforms for an infiltration mission, the captured uniforms are totally out of date, as no one has actually seen a Fire Nation soldier for decades. At least Sokka tips them off before they find out the hard way.
- Drop the Hammer: Several of the Fire Nation soldiers fighting Aang use hammers, which Aang uses against them by having them strike the same spot on a catapult so when he hits it again, the air blast destroys the catapult.
- Dual Wielding: While sabotaging the Fire Nation ships, Aang gets attacked by a soldier wielding two hammers attached by chains.
- Even Evil Has Loved Ones: For the first time, Zuko is affected by and visibly returns Iroh's affection for him. It couldn't be any clearer that he notes the contrast between Iroh (whose son died and who is still mourning him) versus Ozai (whose son is still alive, but cast aside due to the father's arrogance).
- Flight of Romance: Sokka takes Yue on one. The arrival of the Fire Nation navy ruins it.
- Foreshadowing: While said in a Played for Laughs manner, Aang's comment about the spirits bringing forth a giant monster to swat the invasion fleet ends up playing a part in the next episode...
- Honor Before Reason: Yue choosing to go forth with her Arranged Marriage despite knowing he doesn't care about her.
- Instant Armor: More like Instant Snowman— Aang bends the snow around his body to become one.
- Jaw Drop: Sokka's reaction when he learns that Hahn is Yue's fiancé.
- Jerkass: Hahn. He says right to Sokka's face that he's pursuing Yue merely for the perks of being the chieftain's son-in-law.
- Jumped at the Call: Sokka is the first to volunteer for the mission to infiltrate the Fire Nation Navy.
- Like a Son to Me:Iroh: I just nag you because, well, ever since I lost my son...
Zuko: Uncle, you don't have to say it.
Iroh: I think of you as my own.
- The Native Rival: Hahn, to Sokka. With a nice little subversive twist, in that Sokka is a rustic tribesman from a backwater village in an urban setting.
- Noisy Nature: Zuko finds some sealion-turtles that won't stop making noise while infiltrating the North Pole, and yells at them to be quiet.
- Non-Indicative Name: Technically, the Fire Nation Navy is invading the Northern Tribe, not putting it under siege.
- Off-Model: Before departing, the paint on Sokka's forehead disappears right when he turns to look at Yue, then it reappears in the very next scene.
- Oh, Crap!:
- Aang's reaction when, after subduing the first Fire Nation ship, he sees that there are still hundreds on their way.
- Katara when Zuko shows up in the Oasis.
- One-Man Army: Averted. Despite being possibly the most powerful bender in the world, Aang can't single-handedly take on a whole navy, since he's still just one kid.
- Opening Shout-Out: 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.
- Outliving One's Offspring: This is the first episode to allude to Iroh's son Lu Ten having died prior to the series.
- Phlebotinum Battery: "You rise with the moon. I rise with the sun."
- Punishment Detail: Subverted. Sokka expected Arnook to possibly sentence him to some grim punishment such as scrubbing the barracks for his insubordination against Hahn, but no, he needed Sokka to guard Yue.
- Screw This, I'm Outta Here!: Subverted; Yue flees as soon as Zuko shows up in the oasis, knowing that she has no way of fighting him, but she returns soon after with Sokka and Appa.
- Snow Means Death: Black snow, to be precise, caused by the soot of the Fire Navy ships' smokestacks mixing with the clouds.
- Surprisingly Realistic Outcome: As detailed under One-Man Army, Aang is on his way to becoming the most powerful Bender on the planet. But he's still only partially trained and is also an adolescent. So while he's able to inflict his share of damage to the invading forces, Aang's utterly run ragged by the end of the first day of fighting (and the invasion force is so large that even a fully-trained Avatar would have their work cut out for them).
- Take That!: Confirmed by Word of God.Pakku: A couple years and you might be ready to fight a sea sponge.
- Technician vs. Performer: Highlighted early in the episode when Master Pakku is able to progress through dedication, passion and hard work (Technician) while the naturally skilled Aang goofs off (Performer).
- Those Were Only Their Scouts: Aang and a group of Northern Water Tribe benders manage to scuttle the Fire Nation ship that menaces the wall after several minutes of fighting. Then Aang looks up and sees dozens of Fire Nation warships farther down the bay.Aang: You Have GOT to Be Kidding Me!. [flies towards the warships]
- Took a Level in Badass: 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?"
- Trailers Always Lie: The promo chronologically switches the part where Aang goes from "I'm just one kid" to "I'm going to make a difference this time", making it look like there's going to be a lesson (even one person can make a difference). In the episode, it's the other way around.
- Unwanted Assistance: Understandably, Yue is undoubtedly unaware of how the Avatar's power works, which leads her to think that something is wrong with him.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!
- Upper-Class Twit: Hahn is essentially the Northern Water Tribe's equivalent, due to warriors being one of the most respected classes. He even insults Sokka based on her being 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 that the stolen Fire Nation armour is several decades out of date, and he can't get Zhao's name right at all.
- We ARE Struggling Together: Sokka ends up getting into conflict with Hahn, as the former knows Zhao and the Fire Nation a lot better than he does, and calls him out for being a self-absorbed jerk, while Hahn insults Sokka for his ignorance of his tribe's political complexities. The chieftain is having none of this infighting amongst his ranks, and takes Sokka off the mission (though he doesn't pick Sokka for the reason you might expect).
- Worf Had the Flu: 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.