Things are getting really real at Hogwarts this year, folks. In the last installment, Harry and the gang finally got people to realize that Voldemort is back. Which is what Harry has been trying to do since Goblet of Fire, as you may recall.
Unfortunately, Sirius (that's Harry's godfather) died in the process, so Harry's starting off the school year dealing with that. Major bummer. Also, Harry's just heard a super important prophecy linking his birth to Voldemort's eventual downfall. It seems that Harry is going to have to be the one to defeat Voldemort—no one else will do.
Oof. No pressure or anything.
But even "The Chosen One" can't escape from some normal high school stuff, it seems. Harry and his friends are back at Hogwarts and starting to take advanced coursework. In fact, since Snape isn't teaching Potions anymore, Harry is allowed to continue studying the subject with a dude named Professor Slughorn.
Dumbledore wants Harry to get close to this Slughorn guy because the Potions prof has information that's crucial to the whole "defeating Voldemort" enterprise. It seems that Slughorn taught young Voldemort (then known as Tom Riddle) years ago, and he was somehow mixed up in the beginnings of Voldemort's sinister plans to take over the world.
The teacher and his evil student had an important conversation that Dumbledore desperately wants to know more about, and Harry's been entrusted with squeezing Slughorn for the deets.
Meanwhile, we should also mention that something super weird is going on with Draco Malfoy. Like, even weirder than the usual Draco antics. We know he's never been the nicest guy, but things seem to be going to a whole new level this year.
Voldemort has given him a task that involves working directly with the Death Eaters, sneaking around Hogwarts to repair/test a "vanishing cabinet" that's connected to one somewhere outside of Hogwarts' grounds, and (we think) planting poisoned and other dangerous objects around school grounds that are intended for Dumbledore. Pretty vile stuff, even for Draco.
But Snape made an "Unbreakable Vow" to Draco's mother that he'd help the boy succeed on whatever this mission is…so, it's pretty clear early on that nothing good is going to come of this.
Anyway, while all this nefarious behavior is going on, Harry has suddenly become a Potions superstar by following the handwritten margin notes in a used textbook he found in Slughorn's cabinet. Finally, something is coming up Milhouse for Harry.
The previous owner of the book called himself the "Half-Blood Prince" (wink, wink, nudge, nudge). Harry even wins a vial of a super useful potion called Felix Felicis, aka "liquid luck," for brewing the best potion during one of his lessons. (You can guess how much Hermione likes having Harry steal her thunder as the best student in the class.)
Harry ends up using the luck potion to get the facts out of Slughorn that he's been too ashamed to tell Dumbledore (or anyone else) up to this point: turns out he gave Voldemort information that helped him become a lot harder to kill.
See, Tom Riddle came to Slughorn as a student, in what might be described as the creepiest office hours ever, and asked him about Horcruxes.
What's a Horcrux, you ask? Good question: it's an object that can take on part of a person's soul. How does that magic take place? Another good question: through murder (eep). The idea is that if you divide up your soul a whole bunch, it's a lot harder for people to kill you.
Creeped out yet?
Once Slughorn has spilled the beans about his Horcrux chat with Tom Riddle, Harry and Dumbledore realize that Voldemort has likely divided up his soul into a bunch of different Horcruxes and hidden them all around. Like a super disturbing Easter-egg hunt.
Dumbledore had suspected this already, and it turns out a couple are already destroyed. In fact, Dumbledore has a seriously funky injury on his hand from destroying a Horcrux pretty recently. But there are definitely more around. And Dumbledore is only one (slightly mangled) man.
So, Dumbledore invites Harry to travel to a location where he believes another Horcrux is hidden. Things get pretty dicey—as you might expect, the Horcrux has some seriously spooky protections around it—but they manage to nab and escape with the Horcrux.
Success? Eh, not so fast. When they get back to Hogwarts, Dumbledore immediately senses that trouble is on the way. He orders Harry to hide nearby, and sure enough Draco shows up. He's bent on killing Dumbledore. Poor Draco tries to talk tough, but Dumbledore (and we) can tell that his heart really isn't in it.
Poor Draco? What are we even saying right now?
Then the Death Eaters show up. And then Snape.
And then the unthinkable happens: Snape kills Dumbledore.
Harry, who's never liked or fully trusted shady ole Snape, is obviously furious, and he chases Snape from the scene of the crime. To try to catch Snape, Harry decides to use a spell from his precious used Potions book, but Snape easily deflects it. Because—drumroll please— he was the one who wrote the spell in that textbook in the first place.
That's right: Snape was the Half-Blood Prince. See that one coming? Well, Harry sure didn't.
Anyway, Snape flees, and Harry goes back to grieve Dumbledore with the others. As you might imagine,
we're everyone's pretty broken up about the headmaster's death.
Then, just when you thought you could put that Kleenex box away, Harry gets one final bit of bad news: The Horcrux he and Dumbledore rescued was a fake. A thief with the initials "R.A.B." stole the real one. Harry tells the others he won't be returning to Hogwarts next year. Instead, he's going to go hunting Horcruxes.
Alone? Yeah, right: Hermione and Ron are totally going with him. Because friendship.