Supernatural Season 7 premiered on September 23th, 2011 and concluded on May 18th, 2012. It consisted of 23 episodes beginning with the episode "Meet the New Boss" and ending with the episode "Survival of the Fittest".
Season 7 begins with Castiel declaring the he is the new "God" and that if they stay in their place they may live in his kingdom, but if they move against him, he will destroy them. Then he disappears. Sam is fighting the flashbacks from his time in Hell. He starts seeing Lucifer, who tells him that he is still in Hell, and everything else is an illusion. After Castiel kills many of Heaven's angels for following Raphael, and starts killing humans in his quest to punish sinners, The boys and Bobby summon the horseman Death who is the only one who has the power to kill Castiel. Death informs them that he not only swallowed souls, but also swallowed the first beasts, the "Leviatian". They must convince Castiel to return everything to Purgatory before his vessel melts and all of these creatures are released unto earth. Once Castiel figures out that the Leviatian are taking control of him and causing him to kill an untold number of innocents, he agrees to return everything to Purgatory. However, during the ritual, the Leviathian hold on inside of him and take over, releasing themselves into the public water supply and killing Castiel. This gives them the power to go anywhere.
Bobby discovers that the Leviathian are like shapeshifters, but have a taste for human flesh, and nothing can kill them. The leader takes over one of the most powerful men in the country "Dick Roman". They start planning on how to make humans more complacent and more obese in order to provide more meat. They also start planning on how to breed out all undesirable traits and building state of the art slaughter houses for humans. While this is going on the team must figure out how to kill them. Bobby is shot by Dick Roman and killed. Sam's tricks that he has been using to control his mind problems no longer works and he is admitted into a Physc ward because Lucifer will not let him sleep. If they cannot fix this problem Sam will die. Dean tracks down a legitimate "healer" and discovers it is Castiel, who has no memory of his former life. The demon Meg also shows up, and they all go to Sam. Meg ends up telling Castiel who he really is, and he attempts to smite the demons guarding the hospital. He is successful, but during the event, he regains full memory of his former life. He feels that he should have died because of all that he did, but Dean convinces him that maybe he did not die so that he could fix his mistakes. He attempts to heal Sam but discovers that his mind has crumbled to dust and that there is nothing left to rebuild. However, he tries to shift the problems Sam is having into himself and is successful. Sam is cured but Castiel is no longer sane because of what he took from Sam. Meg takes a position as nurse at the hospital to watch over Cas.
Dick Roman digs up a stone tablet that the boys get a hold of, and when the tablet is broke open, Castiel wakes up, and a new prophet is activated. Although Castiel is awake, he is not right. He does inform the boys that the tablet is the word of God, and that Kevin Tran is a prophet. Kevin is the only one who can read the tablet and states that the title is "Leviathian". They also discover that Bobby did not go with his reaper and has remained behind in order to help the boys. However, since all spirits eventually become vengeful, this also happens to Bobby. During a clear moment, he tells the boys to burn the flask that is holding him back, and that when it is their time, to go. Kevin translates the tablet and the boys try to figure out what it means about slaying the Leviatian. Once they figure it out, the ask Castiel to help, but he is still not right, and not willing to fight. He does decide to help, and the boys track down Dick Roman, because killing him is the key to slaying the Leviathian. They succeed in slaying him, but when he dies, he explodes, taking Dean and Castiel with him. Crowley takes the prophet and Meg and Sam is left alone. The season ends with Dean and Castiel being in a strange place. Castiel informs Dean that they are in Purgatory and he disappears.
*In this show, Purgatory is where all of the creatures go when they are killed, such as vampires, werewolves, etc.