Death told Sam and Dean that God had built Purgatory especially for the Leviathan after Castiel had released and absorbed them. He explained that they were the creatures created before the angels and man. God had locked them away because they were so ambitious, so hungry that they would eliminate anything else in creation. God was afraid that the Leviathan would "spoil the rest of the Petri dish."
Thus far, judging by their plans to turn humanity into a herd for slaughter, Death's assertions have been right. Killing them, as Sam put it, has "been a problem." They've been invincible more or less---save from borax and beheading---but either solution is only temporary. If their heads should be near their bodies, they reattach and go on the offensive once again. Their ability to blend into and become anyone gives them an advantage to infiltrate the world in order to conquer it.
"Reading Is Fundamental" finally answers the question: how does one kill a Leviathan?
We start with Kevin Tran, an ambitious and focused high schooler preparing for college. He is dedicated to his studies---so much so that he times his practices and study sessions. Tran is like a lot of over achieving teenagers: stressed and under pressure. He wants a perfect score on the SAT and to get into Princeton. Later, he informs Dean that he wants to be the first Asian-American President of the United States of America. He is going places if he can stick to his regime.
Unfortunately, as Sam and Dean crack open the hunk of clay that Dick had dug up, he is struck by lightening. It stuns him and leaves him flat on the floor to wake in the morning, his life forever and irrevocably changed. Instead of going to school to complete his test, he drives down the road, driven by the need unlocked. The tablet the Winchesters uncovered pulls on him.
The brothers, as they are setting out to figure out what this tablet is, receive a call from Meg. She has been stationed at Castiel's bedside, watching and waiting for the angel to return to the land of the waking. She tells them if they want answers they'll have to come see for themselves---and their plans change.
Upon arrival, Dean remarks that he is not looking forward to seeing what has become of the fallen angel. He is apprehensive. After all, they had left him behind catatonic and facing the horrors that Sam's Hell memories bring. They are uncertain as to what they will find. Entering the room, they find out quickly that he is nothing like his old self.
Castiel is no longer the serious warrior angel he once had been. He appears more like a child than an angel---going as far as to demand that Dean pull his finger so he can blow the light bulb in the room. Even so, he knows what the tablet is---even if he cannot read it. When Meg tries to look at it, still in his grasp, Dean gets into a heated confrontation with her---which causes Castiel to disappear to the day room.
Dean reluctantly agrees to go and talk to him, and we see Castiel sitting forlorn and alone. He pulls out a board game---Sorry---and invites Dean to play. It is a symbolic gesture---and extremely childlike in nature. He can't make what he did wrong entirely right, but he is making a slight effort. Dean implores him to help them with the Leviathan, to join them in the fight and fix that mistake. Instead, Castiel tells him that he no longer fights. He now "watches the bees."
Meanwhile, Sam and Meg have a confrontation of their own, and she prepares to storm out and take Castiel with her. In his haste, Sam leaves the tablet unprotected on the floor of Castiel's hospital room. Once they settle their differences, they learn that the tablet is gone and Sam chases down the thief---none other than Kevin Tran.
It turns out that while Tran excels at academics he also has another unique gift: he can read the Word of God. The tablet specifically describes the Leviathan---and the piece of the puzzle the Winchesters have been waiting for all this time: how to kill them.
This is why Dick was so frightened, so angered, so anxious in "The Girl with the Dungeons and Dragons Tattoo." This is why he was frantic to get it back before the Winchesters could learn its secrets. Until now, he has been unstoppable, and repossessing this object before its secrets can be learned is imperative.
Before he can send his own after it, however, the angels are one step ahead. They arrive to Castiel's hospital room and attack Sam and Meg to get to Tran and the tablet. It is their duty to take Tran, a prophet, to the appointed desert and learn the Word from him as ascribed in their orders. They are about to take Tran and leave the brothers high and dry when Dean and Castiel return to the room.
These angels are from Castiel's Garrison---from another lifetime. Before they can make their move, Dean slaps his hand on a banishing sigil and zaps all of them away so they can make a getaway. They decide to make the trip to Rufus's cabin, Tran in tow rather he wants to or not.
Along the way, the brothers stop to fuel up their stolen car and buy food. Sam spots a news report that lists Tran as missing. If they aren't careful they'll have another issue on their hands. Meg has an issue of her own when two demons spot her. It would seem that there are several roadblocks being put in their path.
They manage to get to the cabin and there, Tran starts to translate the information on it. It gets to him, that this crazy situation is possibly ruining his life, and he starts to hyperventilate. Dean helps him by getting a brown bag to breathe into, telling him that it "sucks to be chosen." He would know. As Tran finally calms down, he sets back to work. It is his only possible way home and to achieving his goals.
Before he can finish, Meg goes to a secret meeting with the demons she met earlier. They want to turn her into Crowley---unless she can give them something better. She offers Castiel, but promptly kills them and returns, only to be trapped in a Devil's Trap.
She explains that she has one cause---and that is to overthrow Crowely. Once upon a time, it had been to do her father's bidding and to release Lucifer. Now it is to depose the King of Hell---probably to place herself on its throne. Her excursion, however, calls the attention of the angels seeking them and they arrive to claim Tran.
Dean is furious and tells them that they need Tran's information so they can "clean up one of the angel messes." Hester won't hear of it, sticking to the orders long ago ascribed for the tablet. She attacks Castiel, who doesn't fight back, only to be stabbed in the back by Meg. Whatever strange relationship these two have, as opposite as their species are, they are most certainly becoming more than allies.
Upon her death, the other angels agree to allow Tran to provide a translation for Sam and Dean, and they allow them to then take him home to guard over them. They make a mistake in doing so, playing right into Dick's clutches as the officer that Sam had seen on the television is none other than a Leviathan himself.
He promptly kills both angels and morphs back into a familiar face: Edgar.
The episode revealed the fundamental flaw of angels: they do not adapt. Unlike the Leviathans, who seem adept at adaptation at dizzying speeds, angels adhere to orders and rigid protocols. Regardless of a long absent father, a thwarted Apocalypse, and the current dire situation, they refuse to budge on plans set into place at the beginning of Creation---or near beginning. The angels insist that they must follow through and take Tran to the desert as preordained once the Word had been written. Nothing seems to make them sway from that---and as Hester takes out her anger on Castiel for deviating from the original plan---it is obvious that those who remain are bitter about this.
Edgar asserts that "rock beats scissors, Leviathan beats Angel," and it is quite literal. Those that adapt continue to live. Those that do not die. The question becomes, then, what will the angels do to respond to the inevitability of change? The threat Leviathans provide is not so much manifested in their more ancient status. It is in their malleability, their evolutionary skill to shift in any situation. As angels were designed to remain obedient to the point of obsession, Leviathan are able to change course and change quickly to fit their needs.
Edlund put a lot of shout outs into this script, harkening often to season 1 itself in its construct. Tran is a reflection of Sam's season 1 self. He is bound for college, has a dream of living a normal life, and is ambitious about his goals. Much like Sam, the supernatural has another plan in mind, and all of these dreams are shattered much like the tablet is when Castiel drops it onto the floor.
Meg, sitting at Castiel's bedside wearing headphones and reading a magazine, is yet another harken back to season 1. It is in this position that Sam first encountered the only demon that has been working against---or with them---since nearly the beginning.
The truck drivers that Meg encounters reflect the possessed driver that had smashed into the Impala to close the first season. Even their semi has the same style cab.
Edlund also sprinkles in other shout outs. Castiel "boops" Tran's nose, much like a drugged out Sam had Dean's in season 5's "Sam Interrupted." Tran being a prophet---and his subsequent reaction---recall Chuck's presence in season's 4 and 5. Chuck didn't want to be a prophet, either, and often reacted with a level of nervous hysterics. Ruby's manipulation of Sam is brought up as a sore spot when the angels arrive and find him with Meg. They assert that the two of them are an item, and Meg quickly moves to refute the claim. Castiel's state in the mental hospital isn't much different than the hunter they encountered in "Sam Interrupted," more interested in playing board games than getting involved in fighting. Sam spotting the news report reflects him spotting the news about their Leviathan doubles committing crimes and being "made" by the clerk.
This episode laid the ground work for what the Winchesters need to do to stop the Leviathan. They have the tools, now it's a matter of executing it. Judging by the preview, that might be easier said than done.
Osric Chau played Tran, and showed an ambitious teenager thrust reluctantly into the world of the supernatural well. He made us sympathize with him as it became revealed that he was as much a pawn in this as Sam and Dean. Unlike the brothers, he had not been equipped to handle this. Even so, Chau showed how Tran dug deep in himself to do what needed to be done. He also showed Tran's naivety in trusting the angels to protect him. An open expression crosses his face as he explains to his mother that they are there to keep him safe, only to morph into one of horror as Edgar kills them.
Rachel Miner returns as snarky Meg. She is as self serving as ever, covering her butt as she goes so she doesn't end up on the losing side---again. Even so, Miner gives Meg a different tone in this episode. She seems to actually care for Castiel, knowing his new ticks and talking to him in a patient manner. Miner shows that while Meg hasn't lost any bite, she's mellowed in some ways, too. Of course, it could be a cover for an elaborate double cross, but Miner makes us want to trust Meg---even when we know we shouldn't. After all, she knows how the Winchesters operate and knows the majority of their weaknesses. Yet, she knows that she also needs them to defeat Crowley. Miner makes us curious to see just what Meg's endgame is and how her survival might affect the rest with high stakes.
Misha Collins returns as a very different Castiel. He is not the awkward nerd angel of the past, nor is he the warrior from before. This Castiel has childlike remorse for what he has done, yet strange insight into the world around him. He can see what is around him with awe and wonder and is easily distracted. Collins showed Castiel's remorse in the childlike gesture of playing Sorry well, even if it didn't sway Dean. His state is tragic, yet brought on by his own actions. Collins takes the character in a new direction without losing the mannerisms. He still tilts his head when he doesn't understand. He still casts his eyes down when he is at a loss. Collins managed to repackage the character in yet another transition that is a total 180 from his stint as a replacement God at the beginning of the season. Not only did Castiel take on these new qualities, the presentation of the character here reflected a very "Flowers for Algernon" style. Castiel knew things, was aware of mistakes, and yet just couldn't put everything together to manufacture a true remorse for those actions. It seemed that he would gravitate between his old self and this new childlike state---much as he expresses his knowledge on the topic and then hear him talk to Meg over the phone about the "unhappy dogs."If anything, it is a true testament to Collins's ability.
Ackles expressed Dean's longing to forgive----even if he couldn't. When Castiel invites Dean to play Sorry, we see Ackles put patience in his features and voice, as he coaxes the fallen angel to fight with them. It isn't until he realizes that there is no chance that Castiel will join them that Ackles uncorks Dean's temper beautifully by swiping the boardgame to the floor. Ackles then shows Dean's patience again with Tran, helping him through his meltdown upon having this thrust upon his shoulders. He demonstrates Dean's determination to do what needs to be done when confronted with the angels demanding they follow orders. His grim expression as he banishes them says it all: there is too much work to be done to have them stand in the way yet again. Ackles also shows Dean's ability to sympathize, as he helps Tran with his meltdown. He is matter of fact and calm, not getting into the boy's face about getting worked up, rather, we see a marked patience about Dean through Ackles. His voice is soft, albeit firm. His line about being chosen is said in such a way that we know that Dean has a profound understanding and empathy for Tran, as he has been in that very spot himself. It is right that Dean guide him here, and Ackles shows just how Dean can help Tran get from upset to focused. As he sits back down to watch over Tran, Ackles makes Dean a comforting presence, just simply by being there.
Jared Padalecki shows Sam's frustration upon learning that Castiel can't simply give them the answers well. It is in his exchange about "Megatron. A transformer wrote this?" Jared makes Sam's face reflect his inner frustration in knotted brows and pursed lips. He then shows his ability to empathize with Tran, knowing how he feels about having his world turned upside down. He is frustrated at having to chase him, but Padalecki shows Sam's sympathy for him when Tran begs him not to kill him. Padalecki shows Sam's gentleness best when he simply asks Tran to open the bag, to see what it is he refuses to let go. He is not judging or demanding. His facial expression becomes soft and open as he gives Tran space, showing that Padalecki knows how to convey with simple body language his character's intentions. He shows his willingness to trust, albeit cautiously, when he allows Meg back out of the Devil's Trap---and Sam's ability to forgive in the soft manner in which he speaks to Castiel about seeing Lucifer. Sam understands what havoc this can wreak, and Padalecki schools his features into a mask of sympathy. As Castiel reveals how it has changed him, ever the one extending the olive branch to help, Sam insists that they will find a way to fix him. Padalecki puts into his body language and his voice all the opposite things than Ackles did into Dean. Sam understands Castiel's predicament, and he puts it best through Padalecki's soft and adamant voice, "I know you never did anything but try to help."
Judging by the preview, it would seem that the brothers need to gather all the components for their spell---including blood from Crowley---and the Alpha Vampire?