So, Cas is totally like super-sized-powered now because he has Leviathans up inside him and he's all, "Back off bitches, I'm gonna stop the beasts in my belly if it kills me," and Dean is all, "Cas, don't do this, I love you, man," and then Cas is all, "Let me right my wrongs, dude," and the Leviathans are all, "Too late, we're gonna bust out of your tummy like the baby alien in Aliens," so Cas runs away and dies in a lake and the Leviathan are all free now and Dean saves Cas' trenchcoat so he can cry and cuddle it every night. In the meantime, Sam is still completely crazy-cakes from Cas busting down that wall in his head in like S6 and now he thinks Lucifer is following him around singing showtunes and shit.
So Cas is dead and Sam is crazy and Dean is all angst-o-rama and then Sam runs off to deal with this other shit. Turns out, when Sam was like 15, he hooked up (in a kind of adorable scene) with this cutie who turned out to be a monster that like has to eat people. Her mom was batshit crazy and Sam had to kill her but his girlfriend, Amy, was all, "I won't eat people, I swear," and Sam let her go. She's all grown up now, working in a morgue and has a kid, who is sick and needs live meat, so she kills people for him but she swears she won't do it again once her kid is all better, so Sam lets her go, again. (hey, why does SPN have a thing about monsters who like to eat people? Stop with all the eating people). But Dean kills Amy anyway and eventually Sam finds out and they won't stop talking about it in S8 because Sam thinks she was a good person and Dean thinks she was a monster and it's all just a metaphor for how they are so different and they can't love each other anymore. Or something. They'll bring it up in S8 many, many times - Remember Amy’s Name.
So then, two of the Leviathans, which are like the baddest motherfuckers Purgatory has to offer, have decided to be in a look-a-like contest with the Winchester Bros and kill some people while they are at it. So, Sam and Dean are back on the FBI's Most Wanted list, but alas, no Henricksen to chase them across America anymore. So, thanks Leviathan dudes.
Also, the boys meet this other hunter, named Garth, who is kind of an obnoxious idiot but apparently Bobby has known him for years and they have a special relationship, even though Sam‘n’Dean have never heard of him before, but whatever - you either love Garth or hate Garth, and you'll see him again in season 8. Remember Garth’s Name.
So, this dude named Dick, who is apparently like No. 1 Supreme Leviathan, is all using this chain restaurant to craftily slip mood suppressors into the food until all the people in the world are just lazy ass slugs that his peeps can then capture and eat. It's like a factory line of fatties for the taking. So the boys have to kill his ass but instead Dick shoots Bobby in the head. Yeah, Bobby's dead.
Also, they meet this teenage girl whose Dad is a hunter and missing and they save the dad and convince them both that this is no life for her. They retire in style, but this is important because Dean bonds with this girl, as Dean does with kids, and she will reappear in S8. Her name is Krissy. Remember Krissy’s Name.
Time After Time is an episode in which Deans goes back to 1944 to help Eliot Ness (played by the gorgeously yum Nicholas Lea) hunt a god or something. Find it, watch it, drool over Dean in period clothes.
Let's not forget, Lucifer is still haunting Sam, singing showtunes and being generally annoying, so eventually Sam ends up in the psych ward. Dean goes to a faith healer to help Sam and it turns out to be Cas - HE’S ALIVE - but with amnesia and a wife (<--that we never hear of again), and Dean is all, "Remember how much I love you? Why can't you remember me?" Or something closer to: Dean doesn't tell Cas the truth but he talks about this guy he knew named Castiel a lot and Meg is there for some reason smirking at them and Cas is like "Yeah, so I'm that guy, Cas, huh? I have amnesia, I'm not stupid, I got it, thanks," and then uses his angel powers to do some stuff and kill some demons and save Sam. But he goes crazy for some reason, idek, and ends up in the mental hospital himself and Meg, wtf even, agrees to stay behind and watch over him because she hates Crowley. Or something.
Then they find out Bobby is a ghost and he is like following them around now helping them out. (Yes, they have a ghost-friend now). Then Dick gets the hard-drive with all the Winchesters information about him on it and gives it to this hacker chick, Charlie (Felicia Day) to crack for him but the Winchesters get to her first and Charlie's all, "Fuck my life, yeah I'll help you, but stop hitting on me, dude, I like boobs." She's kind of awesome and she'll be back too. Remember Charlie’s Name.
Now Meg calls to say Cas is like awake and talking because apparently he was comatose crazy man before - idek - but also the boys have to go help this teenager named Kevin who got hit by a bolt of lightning and BAM, he's a prophet now. Prophet Kevin has a major storyline in S8, so Seriously, Remember Kevin’s Name.
So Sam, Dean, Crazy Cas, Meg, Prophet Kevin, Charlie (maybe?) and Ghost-Bobby all go to kill Dick the dick. Stuff happens and Bobby is like at rest now, I don't actually remember how that happened, but no more Ghost-Bobby, and uh, some stuff and then they kill Dick and he explodes and there is a bright flash of light and when it is over, all the Leviathan are banished to Purgatory and Dean and Cas are missing, like POOF-disappeared. Final shot is Dean suddenly appearing in Purgatory with Castiel and being like, "The fuck?" and then Cas disappears on him and he is all alone and he's like, "Damn, this sucks." End Cut.
When we come back in season 8, Dean has just escaped from Purgatory and Sam is playing house with Some Random Chick and NOT looking for Dean and we don't even know where the fuck Cas is (or Kevin or Meg, for that matter) and then there is suddenly also a dude named Benny, who happens to be a vampire - ooooookay, fine, but give just Benny a chance, okay. Anyhoo, you will get what happened in Purgatory through flashbacks, don't even worry about it. It's all good.