''Founder's Day'' (season 1 finale, pictured)
What happened: We gasped (then squealed) as we watched Damon share a passionate liplock with Elena on her front porch. Only it wasn't Elena, it was Katherine — which we figured out when she went inside and cut off Uncle John's fingers with a kitchen knife and stabbed him in the gut.
Why it's essential: This is when fans truly grasped what kind of twists the writers had in their arsenal.

''Klaus'' (season 2, episode 19)
What happened: Elijah revealed to Elena that The Curse of the Sun and the Moon — which kept vampires from walking in daylight and werewolves from determining when/if they shifted — was fake.
Why it's essential: Until this episode, we thought Klaus wanted to break the curse so vampires could walk in the sun, but there was something much deeper at work. Turned out Klaus's father was a werewolf, and the curse kept Klaus' werewolf side dormant. Once he sacrificed the doppelgänger (Elena), he would become a vampire-werewolf hybrid who could sire his own bloodline.

''The Departed'' (season 3 finale)
What happened: Fearing the end was near for the Salvatore Brothers, Elena finally chose between them and told Damon it would always be Stefan. She, however, was the one who died — with Damon's blood in her system.
Why it's essential: It set Elena up to become a vampire in the season 4 premiere and change the show forever: Vampire Elena has now chosen Damon.
How it's still in play: Season 4 has become about finding a cure for vampirism — and Stefan, Damon, and Klaus all want it for Elena.