Chuck is writing about the history of the Impala, for a manuscript entitled Swan Song. He writes that it is the most important object in the history of the universe.

The boys share a beer in the junkyard at Bobby's. Dean tells Sam that, if it is what Sam really wants, then he will support his plan to let himself become Lucifer's vessel in order to trap him in Lucifer's Cage. Dean asks if this is really what Sam wants and Sam tells him that since he released Lucifer, it’s his responsibility to lock him back up again. They prepare by killing demons and harvesting a huge amount of Demon Blood, which Castiel has said Sam will need to drink to contain Lucifer.

Omens lead Sam, Dean, Bobby and Castiel to Detroit, where Lucifer has always predicted Sam will say “Yes” to him there. As they drive, Castiel sleeps alerting Sam and Dean that his angelic energy is already gone and he is now almost completely human. Sam tells Dean that if the plan actually works, Sam won’t be returning from Hell and he tells Dean not to find a way to bring him back. Sam makes Dean promise that when everything is over, Dean will go find Lisa Braeden and live a normal, apple-pie life.

They arrive in Detroit and Bobby scouts the parameter, informing them that Lucifer really is in Detroit. Sam shares emotional goodbyes with both Castiel and Bobby. Bobby tells him to not give an inch and to fight with all his strength against Lucifer whilst Castiel tells him that he will look after Dean and Bobby and that everything will be fine, although he admits he is lying. Sam drinks the demon blood, and he and Dean go to confront Lucifer.

Chuck continues telling the story of the Impala. He says the Impala has the same things most other cars have and a lot of things other cars don’t have (e.g. the weapons cache) but none of that is important. Chuck says the important things are the things that the Winchester’s put into the car along the years; the army man Sam stuffed into the ashtray in the back, the lego Dean stuffed into the heating vents and the carvings of their initials. Chuck says that Lucifer doesn’t care about the car, but he should.

Sam tells Lucifer that he wants to say “Yes” to him in return for his family’s safety but Lucifer reveals that he knows that they have the Horsemen's Rings and plan to trap him. Sam says this doesn’t change anything and says “Yes” anyway. As Lucifer takes over Sam's body, Dean uses the rings and with an incantation opens Lucifer's Cage. Lucifer, however, is in full control of Sam, and closes the cage, taking the rings and disappearing.

Lucifer and Sam have a talk in which Lucifer tells Sam that he should embrace their union as it was always meant to be this way. He tells Sam that they’re two halves of the same whole, that they were literally made for each other and that the Winchester’s were at best a surrogate family as Lucifer is Sam’s real family. Lucifer gathers all the remaining minions of Azazel who have been keeping watch over Sam his entire life (including a grade school teacher and Sam's prom date, Rachel Nave) and tells Sam that it’s time the two of them “blow off a little steam”. He proceeds to slaughter these demons and asks if Sam is “having fun yet”.

Later, Dean, Castiel and Bobby watch the news, as unprecedented disasters occur across the world - the Apocalypse is beginning. Both Castiel and Bobby seem without hope. Dean calls Chuck who tells Dean the confrontation between Michael and Lucifer will take place at Stull Cemetery near Lawrence in Kansas, where the boys first lived. Castiel tells Dean that there’s nothing he can do and that all that Dean will see is his brother being killed by Michael to which Dean responds that he won't let Sam die alone.

In Stull Cemetery, Lucifer faces Michael who appears using Adam as a vessel. Lucifer tries to dissuade Michael from the fight, blaming their conflict on God, and suggesting they walk away. Michael counters that Lucifer is still blaming everyone but himself and that he hasn’t grown at all. Michael says he must carry out God's orders and that it's his destiny to kill Lucifer. Just then Dean arrives in the Impala, Def Leppard's Rock of Ages blaring. Dean tries to talk to Sam inside Lucifer, but Michael tells him it's not his fight anymore. Castiel and Bobby arrive and Castiel throws a Molotov Cocktail of Holy Oil on Michael causing him to burn up and vanish temporarily. Lucifer, furious that Castiel dared to attack Michael, turns and disintegrates Castiel. Bobby shoots him but Lucifer snaps Bobby’s neck.

Lucifer starts to beat Dean, taunting him, saying that he will kill him slowly and make Sam feel every blow. Dean doesn't resist, just telling Sam he won't leave him. As Sam goes to land another blow, he glances at the Impala and sees the toy soldier still in the ashtray in the backseat, and suddenly all his memories of life with Dean rush at him, allowing Sam to take control over Lucifer.

Sam tells Dean everything’s going to be alright and that he has Lucifer contained for now. Sam tosses the Horsemen's Rings to the ground and recites the incantation to open Lucifer's Cage. He and Dean look at each other one last time, but as Sam goes to throw himself in, Michael reappears. Michael tries to persuade Sam to step back so he and Lucifer can have their battle but Sam moves to jump in anyway and when Michael tries to stop him,, Sam drags him into the cage too.

A bloody and broken Dean grieves for Sam. Castiel reappears and heals him before reviving Bobby. Castiel tells Dean that God revived him and made him better than before. Castiel tells Dean he will return to heaven, where he expects without Michael there will be chaos. Dean angrily informs him that if God returns, Dean will be coming for him next for not being there to save Sam from Hell. Castiel says that God helped them in the end and informs Dean that he got what he wanted in the end, no ever lasting peace like the angels wanted nor Hell on earth like the demons wanted; Just more of the same.

Dean leaves Bobby. Chuck tells us in voiceover that Dean won't see him again for a very long time. Chuck says that all Dean wants to do is either find a way to rescue Sam or die, but he won’t do either as he made a promise to Sam that he wouldn’t. Dean arrives at Lisa Braeden's house, distraught, but trying to hold it together. She comforts him and he mourns in her arms for what he’s lost.

Chuck says that he believes this was a test for Sam and Dean from God and he believes they passed that test because in the end, in the fight between good and evil, angels and devils, destiny and God himself, they chose family. Chuck, now dressed all in white, says that endings are hard, and never perfect, and that nothing really ends anyway. He types "The End" and vanishes. Chuck - it seems, like Enoch & Elijah in the Christian bible - has been taken straight to heaven by God having completed his task as a prophet.

Sometime later, Dean is having dinner with Lisa and Ben. Lisa asks if Dean is okay, to which he quietly responds that he is before settling himself into the dinner and, in turn, a normal life without hunting and without his brother.

Outside Lisa’s house, a street light goes out. Underneath it, watching them through the window, stands Sam...

Source » Supernatural Wiki