Stefan takes drastic measures to find out the real reason Katherine has returned to Mystic Falls, and is shocked when she reveals new secrets about what really happened in 1864. Damon tries a new tactic to resolve his issues with Mason,.... but it proves costly. Tyler pushes Mason to tell him the truth about the Lockwoods. Finally, when Katherine delivers an ultimatum, Stefan and Elena are left with very few options.

Elena decides to take matters into her own hands and offers a tempting incentive to Rose for her help. When things take an unexpected turn, however, Rose calls on Damon to deal with the plan Elena has set in motion. Jeremy’s reckless attempt to help Bonnie retrieve the moonstone lands him in a life-threatening position and forces Stefan to put himself in danger. Bonnie and Luka form a closer connection. Tyler shows Caroline the Lockwood cellar, where she makes a discovery that leaves them both terrified.

Katherine’s mind games are getting on everyone’s nerves, but Damon, Stefan and Elena realize that her knowledge of Mystic Falls history will help them stay alive. Frustrated with her relationship with Matt, Caroline finds a new way to get his attention. Alaric makes a surprising confession to Jenna, and Katherine makes a different kind of confession to Damon. Stefan and Bonnie try to convince Jonas and Luka that they should all be working together, but Jonas’ lack of trust leads to a violent and fiery confrontation.

With the arrival of the full moon, Elena tries to prepare for whatever Klaus has planned and Tyler faces his second transformation. The terrifying events quickly spin out of control, despite an unexpected act of courage. Finally, Damon admits the truth to Stefan about a terrible new development they must face...

Klaus and Stefan arrive in Chicago, where Stefan is reunited with a vampire who hasn't really moved on from her interest in him decades earlier. In a flashback to Chicago in the 1920s, Stefan comes face-to-face with a shocking chapter from his wild past. After Damon gets a tip on Stefan and Klaus' whereabouts from an unexpected source, he and Elena follow their trail. Back in Mystic Falls, Tyler is concerned about Caroline and turns to Sheriff Forbes for help.

On the first day of their senior year, Elena, Caroline, Bonnie and Matt are still reeling from recent events, while Tyler seems to be enjoying everything a little too much. Damon is annoyed by a new houseguest, and everyone is surprised by the arrival of a new student in Alaric's history class. Matt asks Bonnie for help when he realizes he made a serious mistake. Meanwhile, Stefan continues to carry out his latest assignment from Klaus.

Elena is surprised to receive an invitation to a formal ball, and when Damon and Stefan hear the party is being hosted at Klaus’s newly renovated mansion, they both insist on attending the event with her. Caroline and Matt also receive invitations to the ball from unexpected admirers. At the elegant party, Elena learns of a horrifying plan that could lead to numerous deaths and she must decide who she can trust with her new information. Caroline discovers a side of Klaus no one would have suspected. Finally, after an evening of violence and dashed hopes, Damon finds a new way to cope.

Damon and Stefan focus a new project that will bring them closer to destroying Klaus, and they quickly bring Elena, Caroline and Matt in on the plan. After Klaus threatens someone close to Bonnie, she has no choice but to work on the spell he needs. Meanwhile, Klaus and Rebekah find the perfect motivator to convince Finn to cooperate with them, but Rebekah is more interested in her own brutal plan to take revenge on Damon. Stefan’s emotional turmoil leads him to a dangerous confrontation with Klaus and, later, to a heartbreaking moment with Elena.

Klaus acts on his intentions to leave town with Elena, but finds resistance from a surprising new enemy. Bonnie calls on Abby to help her with a difficult spell. As events spiral out of control, Elena is determined to protect Caroline, Damon and Stefan enlist Tyler’s help, while Bonnie and Jeremy take a terrifying risk to make sure her spell is effective. Damon and Stefan have a surprisingly candid conversation about the future.

Elena awakens on the morning after the accident to find the thing she dreaded most has come true – she died with vampire blood in her system and now must go through the terrible transition to become a vampire…or face certain death. Stefan promises Elena that he and Bonnie will exhaust every possible way out of the situation, but Damon is furious with his brother for letting Elena die in the first place. Bonnie pays a terrible price for her attempt to change Elena’s fate, while Damon takes out his frustration and rage on Matt. In a rare moment of empathy, Rebekah is deeply touched by Stefan and Elena’s devotion to one another. Finally, Pastor Young and the Town Council members begin rounding up the vampires and their supporters, leading to an unexpected tragedy.

Shane leads an expedition to a desolate island off the coast of Nova Scotia, where he believes the secret of the cure lies hidden. On the trek to the island’s interior, Rebekah and Elena continue their bitter rivalry, Stefan does his best to keep the peace, and Damon accuses Shane of leading them into a trap. Bonnie and Jeremy try to figure out the message of the Hunter’s mark, while Shane reveals more of the legend of Silas and the witch Qetsiyah, along with his own personal history. Back in Mystic Falls, Tyler confronts Klaus, and Caroline is caught up in the violence that follows.

After a twisted plan left the life of his girlfriend Elena linked to that of his best friend Bonnie, Damon is forced to navigate his new reality without the love of his life. A newly empowered Bonnie has decided that she will be Damon's moral compass and keeps a close eye on him as he, in turn, cautiously keeps tabs on Alaric, following the loss of his fiancée Jo. Meanwhile, as he waits for Caroline to sort out her emotions, Stefan takes an active role in protecting the town from Lily and her family of Heretics, who have wasted no time wreaking havoc in Mystic Falls.

Elsewhere, Enzo struggles to find his place in Lily's new life, and is quickly forced to decide where his true loyalties lie. Finally, with the stakes higher than ever, Matt, who has recently completed his deputy training program, teams up with Stefan and Caroline to put a stop to the Heretics, but an unexpected turn of events leaves one of them in a dangerous position.

After being reunited with her former love Julian, Lily hosts a dinner party to introduce him to Damon and Stefan and to declare peace between her family of Heretics and the residents of Mystic Falls. Damon and Stefan find themselves at an impasse when they realize they have differing views on how to handle Julian's arrival. At the party, Bonnie and Matt uncover a strange mystery involving some unsuspecting residents, while a devastating revelation causes Alaric to reach his breaking point.

In order to lure a vengeful vampire hunter named Rayna Cruz away from his friends in Mystic Falls, Stefan heads to New Orleans, where Valerie has learned of a safe house that can protect him. Once there, he comes face to face with Klaus Mikaelson, who soon becomes suspicious of his old friend's unexpected arrival in his city.

Meanwhile, Enzo, who is working on behalf of a mysterious organization called The Armory, approaches Damon and Bonnie with a plan that could protect Stefan from Rayna. Finally, after learning that Alaric has decided to leave for Dallas with the twins immediately, Caroline offers to accompany them on the trip and finds herself contemplating her own future in Mystic Falls.

The crossover continues on The Originals S03E14 A Streetcar Named Desire (II).