"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 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...