What is harmonic balance
After leaving Jinora's spiritless body in Katara's care, Korra and her friends attempt to break through the Northern defenses in order to reach the Southern spirit portal. Although initially unsuccessful, they manage to enter the Spirit World, where Bumi, Kya, and Tenzin leave to find Jinora's spirit, while Mako and Bolin hold back Unalaq in order to give Korra the time to close the portal. However, Harmonic Convergence starts before Korra can close the portal, and Vaatu manages to break free from his prison inside the Tree of Time.
Kya uses her healing abilities to keep Jinora 's energies flowing while her soul is trapped in the Spirit World .
On Varrick 's battleship, Kya has placed Jinora inside a healing tub and tries to stabilize her condition. Bumi is outside playing a flute for Oogi. Naga. and Pabu. and Korra practices her airbending on a training dummy while Tenzin observes her. Mako has yet to tell Korra the truth about their fight when Bolin walks up to him wearing a coat and eating a pie he found inside the ship. He muses that he misses Varrick and asks Mako if he has told Korra yet. Mako dodges the issue, but Bolin reminds Mako of what he told him, namely do it fast and get it over with. At that time, Korra affirms her conviction to close the spirit portals and defeat Unalaq and Vaatu by firebending the dummy's head off, which lands at Mako's feet. When Tenzin describes their plan of attack by breaking through the Northern forces and meeting up with Tonraq 's army, both Kya and Bumi point out that Tenzin's strategy is more aggressive than the ones he has used before, to which Tenzin says that he will do anything to save his daughter. Bumi begins to tell them another war story, but Tenzin shuts him down immediately. Asami comes out and asks if they are ready, since she had intercepted a distress call from the South.
They arrive at the South Pole and Korra reunites with her mother. She is told that her father was captured and the other soldiers were badly injured. Tenzin asks where Katara is and goes inside. He tells her that Jinora's spirit is trapped and that she has been like this for nearly a week. Katara is asked how much longer can Jinora stay in this condition; she tells her son that she does not know, but notes that Jinora is strong to have lasted like this for so long. Korra is informed that the spirit portal is heavily guarded and that they must reach it on their own. Bumi tells them that they must attack from above by retelling an adventure he fought in with a catapult, but Tenzin furiously orders him to stop with his ridiculous lies and be serious because Jinora's life is at stake. However, Asami points out that he might be on to something. She tells them that they have Oogi as well as a plane on Varrick's ship. They split into separate groups; Tenzin, Bumi and Kya will go with Korra on Oogi to get into the portal while Bolin and Mako will go with Asami and create a diversion.
Unalaq is meditating inside one of the tents when Desna and Eska arrive and ask why they are still in the South Pole. Desna points out that the rebel army was subdued and that the two Water Tribes are now united, while Eska adds that the two portals are open and that they should go home. Unalaq says that there is still one thing left to be done that will make all of these achievements "pale in comparison". The twins look at each other while Unalaq describes his plan for a new world that he will rule. He dispatches them to assemble the army to defend the portal when Korra arrives.
Despite their plan, Bolin, Mako and Asami find the army assembled and ready for them. Asami maneuvers through the icicle spikes that are being hurled at them, while Mako provides some cover. Bolin joins in soon after, using explosive packs created by Varrick to punch through the Northern defenses. As Oogi approaches the portal, he is soon attacked and numerous spirits attach themselves to him, bringing the bison down. Korra, Kya and Bumi attempt to fight off the spirits dragging Oogi down. Bumi falls off the
bison and drags down a spirit with him and Oogi crashes into the ground knocking them senseless. As Korra fades into unconsciousness, she sees several Northern soldiers approaching. Meanwhile, Desna and Eska both combat the plane and are able to bring it down.
Unalaq enters the tent where Team Avatar is held captive.
They all wake up and find Tonraq in the tent with them. Unalaq mockingly congratulates them by saying they have gotten themselves "front row seats to the beginning of the new world order". Korra pleads with him to reconsider, but Unalaq ignores her. He denounces her declaration that Wan made the right decision in banishing the spirits from the world and says that it has brought nothing but chaos. Unalaq also rejects the Avatar's role as bridge between both worlds, believing that spirits and human should live together as one. Korra promises that she will put Vaatu back in his prison if he is released and Unalaq reveals his ultimate goal: When Wan fused with Raava. the balance was tipped in her favor. Unalaq plans to rectify this and fuse with Vaatu during Harmonic Convergence and become a Dark Avatar.
Tonraq weakly asks his brother to reconsider this plan to throw away his humanity and the lives of his children to become a monster, but Unalaq brushes it aside, stating his future form will be no different than that of Korra's, with the exception that he will be fully in tune with his spiritual side. He leaves and tasks Desna and Eska to make sure the captives do not escape, promising that he will be back for Korra once Harmonic Convergence is over. When Tenzin reflects that Jinora's spirit is lost for good, Kya reminds him that Bumi is still out there.
When dark spirits cause his mecha tank to go haywire, Bumi accidentally saves the day by destroying the entire Northern encampment by himself and freeing his siblings and Team Avatar .
Bumi emerges from the snow and tries to fight the spirit that he pulled off Oogi. Nothing he does works against the spirit until he pulls out his flute. The spirit, puzzled, stops and gazes at him. Noticing this, Bumi begins to play and the spirit proceeds to calm down. Bumi continues playing, tames the spirit, and reunites with Naga and Pabu. He subsequently sneaks inside the camp around the portal and begins to play his flute once more to tame the other spirits. This time, though, the spirits attack him and he hides inside a mecha tank. The spirits get inside and proceed to take control. Bumi tries to fight them off from inside and is eventually able to escape. He is delighted to find that he was able to destroy the entire Northern camp around the portal during his struggle.
Back at the tent, Korra tries to reason with her cousins, but they refuse to help. She reminds them that Unalaq will not be their father anymore and that they will not be safe from his wrath. Desna firmly denies this and stands behind his father's beliefs that this is right. Suddenly Bumi flies through, proclaiming that help has arrived. The twins approach him but Naga knocks them out and Bumi frees his friends and family. Tenzin is amazed with what Bumi did and asks how he was able to do this; Bumi begins to answer but stops, claiming that they "would [not] believe [him] anyway".
Upon reaching the portal, Korra has Asami take Tonraq back to Katara. She splits up those remaining into separate groups: Tenzin, Bumi and Kya will go find Jinora while Bolin and Mako will engage Unalaq and Korra will go to close the portals. When asked if they might become trapped inside the portal if Korra fails, she hesitates to answer and tells them to move. They make it inside and Unalaq directs the spirits toward them. Korra sends Tenzin and his siblings off to find Jinora and enters the Avatar State. Mako and Bolin hold back Unalaq while Korra reaches the Southern portal. She tries to close it but is blown backward by a pulse of energy as Harmonic Convergence begins. Unalaq looks on with a crazed smile as a purple haze stretches across the poles. Republic City. and eventually the entire world. Vaatu uses the amplified spiritual energy to break free from his prison and laughs triumphantly as he approaches Korra.Source: avatar.wikia.com