"And The Winner Is..." has Amon and the Equalists attacking the Pro-Bending arena, beginning the war against the Equalists
"The Aftermath" reveals that Hiroshi Sato is supplying the Equalists with weapons.
"When Extremes Meet:" Tarrlok arrests Mako, Bolin, and Asami, then reveals he can bloodbend without a full moon and kidnaps Korra. Then we see a flasback to the original Gaang trying to subdue a bloodbender named Yakone.
"Out of The Past:" Tarrok kidnaps Korra and imprisons her somewhere far away, and she contacts Aang and finds out all about Yakone. Meanwhile, Lin breaks Mako, Bolin, and Asami out, and they, along with Tenzin, infiltrate an Equalist base. Then we find out that Tarrlok is Yakone's son. THEN Amon attacks Tarrok, is unaffected by his bloodbending, debends him, and inadvertantly allows Korra to escape
"Turning the Tides:" Amon starts a full-scale attack on the city. Tenzin and his family escape, but only because of a Heroic Sacrifice from Lin. Then, at the end, we see General Iroh leading the United Forces.
"Skeletons in the Closet/Endgame:" We find out that Tarrlok is Amon's brother, and we find out their tragic backstory. Then we have a big final battle with Amon and the Equalists, and Korra is debended! Then Korra unlocks her airbending, and defeats Amon. He and Tarrlok try to escape, but Tarrlok blows up the boat and they both die. Then Aang restores Korra's bending, and she is able to restore Lin and the others as well.
"Civil Wars, Part 2: We get a confirmation that Unalaq is evil, and that he was responsible for Tonraq getting banished years ago. Korra breaks Tonraq out of prison, and the North and South are basically at war by now.
"Peacekeepers:" Mako and Korra break up at the end. Then Korra leaves for help from the Fire Nation, and gets attacked by Eska and Desna, then eaten by a Dark Spirit.
"Beginnings: Parts 1and 2" are not as big as the others, but they change up things by revealing the existence of Vaatu and revealing that opening the spirit portal may not have been such a good idea.
"The Guide" also isn't so major, but it ends with Mako getting framed and arrested. In addition to Vaatu making his present-day debut, the episode also reveals that not only is Unalaq evil, he is The Dragon to Vaatu.
"A New Spiritual Age:" Unalaq reveals he can destroy souls and threatens to do this to Jinora to make Korra open the other spirit portal. Then Korra returns to the normal world, but Jinora does not.
"A Breath of Fresh Air": In the very first scene alone, Bumi can Airbend. As time goes on, Airbenders are popping up EVERYWHERE and they were all formerly Non-Benders. Even one of our villains is an Airbender.
"Rebirth of a Nation": Korra's Harmonic Convergence is revealed to have all but crippled Republic City, and she is subsequently exiled from it.
"The Earth Queen": The Dai Li are still functional and are gathering all of the Airbenders up to make an army for the Earth Queen under HER orders.
"The Metal Clan": Lin Beifong has a half-sister, and the mysterious new 'recruit' at the Northern Air Temple is actually a newly-shaved Zaheer.
"The Terror Within": Aiwei is revealed to be in league with Zaheer, and the Red Lotus came dangerously close to kidnapping Korra.
"The Stakeout": The Red Lotus' ultimate goal is revealed: They intend to assassinate all the world leaders, sending the world into anarchy so they can create a new society from the remains. Also, the Earth Queen declares open rebellion on Republic City by abducting Korra and Asami.
"Long Live The Queen": The Red Lotus meets the Earth Queen and kill her. They then give absolute freedom to the people of Ba Sing Se to do whatever they want. The city promptly begins to spiral into chaos.
"The Ultimatum": The Red Lotus reach the new airbenders, and take them hostage to lure out Korra, threatening to kill them if they don't cooperate. Tenzin and his siblings try to fight back, but after Bumi and Kya are defeated, Tenzin alone can't keep up with all four members of the Red Lotus, and is beaten senseless.
"Enter The Void": P'Li dies in a combustion attack gone wrong, leading to Zaheer going over the edge, literally and spiritually, and finding the ability to fly. Meanwhile, Bolin figures out how to lava bend. However, Korra is captured, and is about to be poisoned, which will trigger the Avatar state. When that happens, the Red Lotus will kill her and end the Avatar cycle forever.
"Venom of the Red Lotus": Oh boy, here we go. Mako kills Ming-Hua with lightning. Ghazan brings the cave down on himself. In the end, while Korra does survive the poison with Suyin's metalbending, she is left in a wheelchair with no knowing how long it will take for her to recover, and it is clear that her ordeal has left her utterly broken both physically and mentally. Jinora is granted her tattoos and is deemed an airbending master. Her hood is removed to reveal she looks just like Aang...
"After All These Years": It's been 3 years since we (and Republic City) last saw Korra. The Earth Queen's sleezy nephew Wu is preparing to take the throne while Kuivira is rapidly bringing the Earth Kingdom under her own rule. Bolin's relationship with Opal is strained because he's working for Kuivira (for whom she and her mother appear to have a strong distaste), which becomes worse when it's revealed Opal's older brother and Kuivira are engaged. Everyone else on Team Avatar is awaiting Korra's return that evening, and joyfully greet Tonraq...who's been under the impression that his daughter was already there...for the last 6 months. We then see her getting humiliated in an underground Earth Bending ring, à la Earth Rumble 6, and denying that she's the Avatar.
"Korra Alone" The reason for Korra's self-induced exile is revealed. She's trying to reconnect with Raava while being haunted by a dark vision of herself. Also, the episode ends with Korra discovering that Toph is alive and well.
While everybody could see the "twist" coming in "The Coronation" where Kuivira overthrows Prince Wu and seizes control of the Earth Kingdom, it's the way she does it that's shocking. She hijacks Prince Wu's own coronation ceremony to announce her formation of the Earth Empire directly in front of the other world leaders, and flat out states that she will destroy any nation that attempts to cross her borders or interfere with her plans.
"Enemy at the Gates" has the reveal of how Kuvira came to power, that she's not so kind to the villages that join the empire as she seems with things like forced labor and reeducation camps, and Varrick's work with the spirit vines will make a superweapon that even he thinks is too dangerous to create. In the end Varrick is forced to keep working on the weapon, Zhu Li betrays him, and Bolin is being sent to one of the aforementioned reeducation camps. Meanwhile Su and her sons Wei and Wing are going to sneak into Kuvira's camp in an attempt to "end this for good".
"The Battle of Zaofu" has the Korra and Kuvira fight for the fate of Zaofu. Most of the episode has Korra losing the battle while Kuvira easily avoids all of Korra's attacks. In the end, Korra is forced to go into the Avatar State. Just when it seems that she is about to crush Kuvira, Korra sees her own face on Kuvira, which forces Korra out of the Avatar State. Just as Kuvira is about to kill Korra, Opal and Jinora attack, resulting in Kuvira ordering her army to attack the city, forcing Tenzan's kids, Opal, and Korra to retreat. During all this, Varrick and Bolin are forced to work with Baatar Jr. to create the superweapon, but Varrick instead turns it into a bomb and forces Baatar Jr. as well the rest of the men loyal to Kuvira to leave the train while he and Bolin stay inside with the bomb. Bolin manages to get him and Varrick off the train before the bomb goes off. The episode ends with Kuvira conquering Zaofu. She speaks to Baatar Jr. who tells her that he can replicate what Varrick did, but will need an assistant. Kuvira gives Zhu Li the task of helping Baatar Jr. and then orders that Zaofu's Domes be dismantled.
"Kuvira's Gambit" reveals a 25-story mech with an insanely powerful spirit weapon attached to its arm as Kuvira's ultimate war machine... and to top it off, she fires the thing into the warehouse she has confirmed her fiancee is being held hostage in.
"Day of the Colossus"/"The Last Stand" is a doozy. Hiroshi dies giving the rest of the team a way to stop the Colossus, a new spirit portal is created in the center of Republic City, Varrick marries Zhu Li, and Korra ends up up with Asami at the end, which is not only a wham for the show, but for children's programming in general.