Sirius finds Kreacher in the attic, but Harry is uneasy, since Kreacher seems strangely docile. Sirius becomes more and more sullen and Harry isn't excited to leave him alone and return to Hogwarts and Umbridge. On the last day before the children return to Hogwarts, Mrs. Weasley calls Harry to the kitchen to speak to Snape. Sirius is there as well, and both men look furious. Snape taunts Sirius about not being able to do anything useful, and then tells Harry that Dumbledore wants him to teach Harry Occlumency, which will protect Harry's mind against "external penetration." Harry wonders what he did to deserve private lessons with Snape.
The order to take private Occlumency lessons from Snape appears, to Harry, to be another snub from Dumbledore, given that Dumbledore is aware that neither Harry nor Snape like each other. Dumbledore is thus forcing Harry to do something that he knows he will hate, without giving enough background as to why Harry has to do this.
Sirius tells Snape that if he gives Harry a hard time, Sirius will know about it. The two argue about whether Snape has truly reformed and Sirius's choice to accompany Harry to King's Cross. Harry puts himself between the two angry men as the Weasley family walks happily into the kitchen. Snape lowers his wand and leaves, while Sirius turns his attention to congratulating Mr. Weasley on his recovery. That evening, Harry isn't able to comfort Sirius at all.
Harry is forced to act like the only adult in this situation and separate Snape and Sirius, which makes Harry feel as though he has to be mature and shouldn't give Sirius a reason to worry. This in turn means that because Harry feels he needs to shield Sirius, he'll be less likely to ask Sirius for help if he needs it.
The next morning, Sirius pulls Harry aside and gives him a wrapped package. He says it's a way to communicate so that Harry can tell him if Snape's being horrible, but Harry knows that to keep Sirius safe, he can't use it. Sirius, Mrs. Weasley, and Mr. Weasley wave the children out the door with Lupin and Tonks and Lupin summons the Knight Bus. Harry, Ron, and Hermione sit on the top level of the bus with Tonks and barely manage to keep their seats as the bus leaps around the country. Finally, it arrives in Hogsmeade and drops them at the gate to Hogwarts. As Lupin tells Harry goodbye, he says that Snape is great at Occlumency and can teach Harry to protect himself.
As expected, Harry now feels as though he has to shut himself off from asking Sirius for help in order to keep Sirius safe, something that Harry shouldn't feel compelled to do. When Lupin also supports Harry learning Occlumency from Snape, it makes Harry feel as though there's something he doesn't know or understand about this situation, given that Lupin and Snape don't have a positive relationship either.
Snape is awful to Harry in Potions. Harry feels even worse when Zacharias Smith acts superior because Harry has to tell him that he has Remedial Potions, Snape's cover story for Harry's extra lessons. When Cho approaches Harry in the hallway, Hermione drags Ron away. They awkwardly discuss their holidays and it takes Harry a minute to understand why Cho mentions the Valentine's Day Hogsmeade trip. He asks her to go to Hogsmeade with him.
Even if Snape is doing Harry a favor according to Dumbledore and Lupin, his choice to create a cover story that makes Harry a joke among his classmates tells Harry that this isn't going to change anything between them or improve their relationship.
Harry knocks on Snape's door later that evening. As they begin, Snape tells Harry about Occlumency, which seals a person's mind, and about Legilimency, the ability to "extract feelings and memories" from another's mind. Snape explains that Voldemort is skilled at this, and while a person usually needs eye contact to perform Legilimency, Harry and Voldemort seem to be connected to each other when Harry is asleep and vulnerable. He says that Voldemort now seems aware of this connection. Harry begs Snape for more information, but Snape refuses to tell Harry anything except that Dumbledore wants Harry to be able to protect himself in case Voldemort tries to access Harry's thoughts or make him do things.
Notice that while Harry doesn't like or trust Snape, the fact that Snape is literally the only adult in Harry's life who's willing to give him reasonable information about what's happening to him means that Harry is dying to know more. While Snape isn't actually double agent here, this shows that Harry's desire for information is so great, he could be willing to trust people he really shouldn't as he searches for more information from people willing to share.
Turning to the Pensieve on his desk, Snape pulls a few thoughts out of his head and puts them in the basin. He then asks Harry to draw his wand and do whatever he can to defend himself while he tries to break into Harry's mind. When Snape says "Legilimens," Harry is unable to fight and sees childhood memories flash through his head. When a memory of kissing Cho flashes through, Harry panics and produces an inadvertent Stinging Hex. Snape tells Harry to close his eyes and empty his mind before trying again. Harry sees the memory of Cedric dying before he throws Snape off again. Snape snarls that Harry needs to discipline his mind. As Harry's memories flash through again, he sees one of running with Mr. Weasley to his hearing.
While the reader, like Hermione, knows that Dumbledore likely has a very good reason for Harry to be learning Occlumency from Snape, the fact remains that this is clearly a traumatizing experience for Harry. Snape is invading Harry's privacy and when the memory of Cedric flashes through, it forces Harry to relive the evening in the graveyard yet again. None of this makes it easier for Harry to learn what he's supposed to be learning, which suggests that trust and a safe environment are essential to education.
Harry triumphantly says that he figured it out, and Snape stares at him curiously. Harry realizes the corridor he's been dreaming about is in the Department of Mysteries. He asks Snape what's there. Snape looks unnerved and refuses to say, and he tells Harry to return on Wednesday. Harry finds Ron and Hermione in the library and tells them that whatever Voldemort's after, it's in the Department of Mysteries. They realize that that's where Sturgis Podmore was likely trying to break in, and Ron says that according to Mr. Weasley, nobody knows what the people who work there do.
The fact that Snape is uncomfortable with Harry's question tells Harry that it likely has to do with the Order, and isn't something that non-members can or should know about. Because nobody in the general public knows what's in the Department of Mysteries or what employees there do, Harry is again unable to effectively figure things out—and instead, must come to his own questionable conclusions.
Hermione and Ron lead Harry back to the common room, where Fred and George demonstrate their new Headless Hats. The hats impress Hermione, but Harry still feels ill after the Occlumency lesson and decides to go to bed. After taking a step inside of the dormitory, Harry suddenly feels like his head is being sliced in two and he hears maniacal laughter. He comes to on the floor, Ron next to him. Harry says that Voldemort is happy; something good has happened.
Hermione's willingness to admit that Fred and George are skilled wizards shows that wizards don't actually need high test scores to create interesting, successful things. Harry's insight into Voldemort's mental state shows him clearly that his Occlumency lesson wasn't effective. The fact that Voldemort is so delighted is also extremely ominous.