Here is the fifth installment in our six-partsynopsis of Harry Potter and the Half-Blood Prince, the sixth book in the series — your essential cheat-sheet for diving into the seventh and final book, Harry Potter and the Deathly Hallows. See chapters 1-5 summarized here, 6-10 summarized here, 11-15 summarized here, and 16 -20 summarized here.
Chapter 21 - The Unknowable Room
Harry is with Hermione and Ron, trying to figure out how to get the memory from Slughorn. He looks in his annotated potions book to see if there is anything that might help and stumbles upon a spell, Sectumsempra, which has a note by it that says "for enemies." He makes a note to try it out later. Kreacher and Dobby Apparate into the room to give their report on Malfoy. Kreacher is useless reporting on mundane things, but Dobby tells Harry that Malfoy has been going up to the seventh floor with a variety of students. It occurs to Harry that Malfoy is visiting the Room of Requirement (see Book 5) which doesn't show up on the Marauder's Map. He also figures out that Malfoy isn't going there with all different students, it's just Crabbe and Goyle taking the Polyjuice potion to transform into different students to keep watch. Hermione notes that he won't be able to get into the room if he doesn't know what Malfoy needs the room for. The next morning Harry is still obsessing over how to get into the Room of Requirement, but Hermione reminds him that he needs to work on Slughorn's memory. Harry goes up to the seventh floor and spend hours there, trying to get the Room of Requirement to appear, but without success. He's late to Snape's class where the students learn the difference between Ghosts (an imprint of a departed soul left upon earth) and Inferius (a reanimated corpse or zombie). Harry and Ron go to the bathroom, where Mrytle pops out of the toilet and tells them about a boy who comes to the bathroom to cry and talks about how lonely he feels and how he has no one to talk to, but Myrtle won't tell them who it is. Ron and Hermione head off to Hogsmeade to practice Apparition (Harry isn't quite old enough to take the test with everyone else) so Harry goes to the seventh floor again to try to find the Room of Requirement wearing his invisibility cloak. He bumps into Tonks who is looking for Dumbledore. She seems out of sorts and leaves. Ron and Hermione get back from practice and Harry tells them about his encounter with Tonks. Harry voices his suspicion that Tonks was in love with Sirius, though Hermione doubts it.
Chapter 22 - After the Burial
Harry is a frustrated that he can't figure out what Malfoy is up to and still has made little progress on the Slughorn issue. The children receive a tear-stained letter from Hagrid that says Aragog has died and that he hopes they can make it to the burial. They are not too keen on going since it's risky to go out at night, and Harry ought to be working on Slughorn. Then it occurs to Hermione and Ron that Harry should take a little Felix Felicis and talk to Slughorn. Harry was hoping to save it for another occasion (involving Ginny and Dean breaking up and Ron being happy for him to date Ginny). Ron and Hermione head off to their Apparition Test and Harry heads off to Potions class. Harry hopes to catch Slughorn at the end of class but Slughorn hurries off before Harry can get a word in. Hermione and Ron return, and though she's passed the test Ron missed by half an eyebrow. Hermione chides Harry for not taking the Felix Felicis before Potions class; realizing he must take it, Harry decides to swallow a mouthful before visiting Slughorn's office after dinner. Harry takes a gulp and an overwhelming sense of luck come over him. He decides to visit Hagrid instead. Hermione and Ron panic and tell him he should see Slughorn. Harry brushes it off and says he knows what he's doing. He grabs his invisibility cloak and heads off, knocking into Ginny on the way out (which she blames on Dean who is right behind her). Luckily, the castle door is unlocked and Harry sees Slughorn on his way to Hagrid's. Harry reveals himself to Slughorn and invites Slughorn to Aragog's burial. Slughorn is hesistant at first, but when he hears that Aragog is an acromantula and that he can possibly acquire the some of the venom, he has a change of heart and runs back to the castle to get some drinks to toast Aragog. Hagrid is a mess as he prepares to bury his friend. Slughorn shows up and before Hagrid buries Aragog, Slughorn discreetly bottles some of the venom when Hagrid isn't looking. Slughorn says a few respectful words and Aragog is buried. They go back to Hagrid's cottage and Slughorn and Hagrid commence drinking. Slughorn and Hagrid get very drunk and Hagrid starts talking about the loss of his parents. Harry then proceeds to tell Slughorn about the death of his parents which greatly pains Slughorn since Lily, Harry's mum, was one of his favorite students. With the combination of Slughorn's guilt over the memory, his drukenness and the luck of Felix Felicis, Harry manages to coax the memory out of Slughorn.
Chapter 23 - Horcruxes
Harry returns to the castle and goes to Dumbledore's office to inform the headmaster, who is both happy and impressed to see that Harry has completed the task. They pour the memory into the pensieve and jump in. They are back in the same memory as before. Slughorn is surrounded by a group of boys including Tom, whom everyone admires. Slughorn tells Tom that he has excellent contacts at the Ministry and that he will go far. Tom says he doesn't think politics suits him and that he doesn't have the right background. It's 11 pm and Slughorn starts hustling everyone out of his office and back to their dormitories. Tom hangs around and asks Slughorn about the Horcruxes, innocently claiming he stumbled upon the term in a book somewhere. Slughorn says that it would be hard to find any books about Horcruxes in Hogwarts and that they are very, very dark magic. Tom carefully flatters Slughorn into revealing that Horcruxes are objects in which a person has concealed a part of their soul. Tom inquires further. Slughorn says that one would split one's soul and hide part of it in an object so that even if one's body were to be killed, one cannot die because part of the soul remains earthbound and intact in the Horcrux. However, that existence is not pleasant. Tom asks Slughorn how one splits one's soul. Slughorn says that splitting one's soul is an act against nature and that it would take an act of extreme evil -- murder -- to rip one's soul apart, at which point a spell is used to encase the piece of the soul in the object. Tom asks Slughorn about the use of more than one Horcrux; for example, splitting the soul into (the magical number) seven parts. Slughorn is alarmed at the thought and starts feeling troubled; he tells Tom to keep quiet about what they have talked about. Tom leaves the office with an expression of wild happiness on his face and the memory ends.
This last memory confirms what Dumbledore had suspected, and he notes that even at a young age Tom desired to become immortal. Harry asks Dumbledore if Voldemort succeeded in making a Horcrux, and Dumbledore confirms it -- Tom Riddle's diary (see Book 2) was a Horcrux. He also mentions that Voldemort was interested in creating more than one Horcrux. The diary was intriguing to Dumbledore because in addition to storing part of Voldemort's soul, it also had a malicious intent in manipulating it's owner to unleash the Slytherin monster. However, the obvious nature of the diary suggests that Voldemort made more than one Horcrux in case of destruction. As Voldemort made more and more Horcruxes, splitting his soul again and again, he seemed to grow less and less human. Harry asks Dumbledore why Voldemort didn't just get a Sorcerer's Stone. Dumbledore think thats Voldemort didn't like the idea of having to rely on drinking an elixir every day and that the Horcruxes appealed to him more, as he would be immortal with or without human form. Dumbledore concludes that Voldemort made seven Horcruxes, one of which resides in Voldemort's own regenerated body. Harry destroyed one which was the diary and and Dumbledore destroyed another which was the black stone ring (Marvolo's ring) which Dumbledore had acquired before picking Harry up from the Dursleys'.
That was what happened to Dumbledore's arm : he was was injured destroying the ring Horcrux, and Snape helped stop the curse from taking more than his right hand. Dumbledore had found the ring in the Gaunts' old house. So now, knowing that two have been destroyed and the last piece resides in Voldemort himself, four remain at large. Dumbledore reminds Harry that Voldemort preferred valuable, magical objects so Voldemort chose his Horcruxes with care. Harry figures out that the Slytherin locket and Hufflepuff cup that Voldemort had stolen are two more Horcruxes. Dumbledore suspects that Voldemort wanted to secure relics from Ravenclaw and Gryffindor as well but he doesn't know what Voldemort could find of Ravenclaw and the one known relic of Gryffindor (the big ruby encrusted sword) is secure. Harry and Dumbledore suspect that when Voldemort had come back to Hogwarts for a job, that his true intention was to find more Horcruxes. Dumbledore is fairly certain that Voldemort cannot have both something from Ravenclaw and Gryffindor, and thinks that Voldemort's sixth Horcrux is his pet snake Nagini (though animal Horcruxes are not advisable). Dumbledore thinks that Voldemort was probably trying to make his final Horcrux with the death of Harry, but that did not succeed so Nagini was the next best thing. So in total there is the Diary (check), Marvolo's ring (check), the Slytherin locket, Hufflepuff's cup, Nagini and something that belonged to either Ravenclaw or Gryffindor (plus the seventh piece in Voldemort's new body). Dumbledore has been leaving school for those long stretches of time to look for these items and he tells Harry that he will bring Harry along to destroy the next one. Harry wonders if Voldemort can feel it when a Horcrux is destroyed but Dumbledore suspects not since Voldemort has been so far removed from his soul pieces. For instance, Voldemort was not aware that his diary was destroyed until someone told him. So Harry figures that destroying all the Horcruxes would get rid of Voldemort. Dumbledore warns him that even destroying the Horcruxes does not diminish Voldemort's magical or mental powers. He also reminds Harry that what makes Harry most different from Voldemort is Harry's ability to love. Harry argues that he must fulfill the prophecy but Dumbledore argues that regardless of the prophecy, Harry wants to kill Voldemort. And for that--the motivation Harry has inspired by his love for his family--Harry will have the upper hand over Voldemort.
Chapter 24 - Sectumsempra
Harry fills Ron and Hermione in on everything that happened with Dumbledore. He also finds out that Ron and Lavender, and Ginny and Dean have all broken up. He's very happy, but is worried about what Ron would think if he started dating Ginny. Katie Bell is back from St. Mungo's and everyone is happy. They ask if she remembers anything from the incident with the necklace and she says she doesn't remember a thing. Harry begins to wonder if maybe Crabbe or Goyle had taken the Polyjuice potion and staked out in the girls' bathroom at the Three Broomsticks to give the necklace to the unlucky victim. He contemplates using the Felix Felicis potion to get lucky and find the Room of Requirement and figure out Malfoy's plans, but Hermione tells him that is useless and a waste of potion. So Katie Bell is back on the Quidditch team, leaving Dean Thomas out. Practice commences as the team has a championship game against Ravenclaw. A few days before the big match, Harry is looking for Malfoy on his Marauder's map and notices that Malfoy is in the bathroom. He goes into the bathroom and hears Malfoy crying to Myrtle, who is trying to help him. Malfoy keeps saying that he needs to do something and if he doesn't get it done, someone will kill him. Malfoy looks up and sees Harry and starts shooting hexes at him. Before Malfoy can cast anymore curses, Harry casts the Sectumsempra spell at him (the spell in his potions text). Malfoy falls to the ground spurting blood from the deep cuts from Harry's spell. Snape runs in and says a bunch of counter curses to stop Malfoy from bleeding and to heal his wounds. Snape sends Malfoy up to the hospital and demands to know where Harry found that spell. Harry lies and said he read it somewhere he can't remember. Snape calls out his lie and tells Harry to bring his school bag. Harry runs back to his dormitory, quickly swaps his Potions text with Ron's and as he's thinking about where to hide the Prince's annotated text, the Room of Requirement appears. Harry runs in and hides the text in a room full of junk. He bring his bookbag to Snape who asks Harry if the Potions book belongs to him. Harry lies and says yes. Snape calls him out again and gives him detention every Saturday until the end of the term, effectively leaving Harry out of the Ravenclaw vs. Gryffindor Quidditch match. Hermione once again chides Harry for using the Prince's textbook. Ginny tells Hermione to quit it and that if Harry didn't have the book Malfoy would have used the Unforgivable Curse on him. Harry is quite delighted that Ginny jumped to his defense. Saturday comes and Harry goes to Snape's office to reorganize and rewrite all the records of Hogwarts' wrongdoers and their punishments. Meanwhile the Quidditch match takes place. Detention finally ends and Harry runs back to the Gryffindor tower to find out what happened. Gryffindor won and Harry kisses Ginny (not a peck on the cheek, but full on snogging). He looks over to Ron, who shrugs his consent, and Harry runs off with Ginny.
Chapter 25 - The Seer Overheard
Harry is exceptionally happy now that he's dating Ginny. Hermione has been doing some research on the Half-Blood Prince and she finds a girl named Eileen Prince who attended Hogwarts and thinks that this girl might have a connection. Harry brushes it off and says that the Half-Blood Prince cannot be a girl. Hermione leaves in a huff to do more research on the Prince. Meanwhile, Harry gets a message from Dumbledore to come meet him. On his way, he bumps into Professor Trelawney who is drunk on cooking sherry. She was trying to get into the Room of Requirement because she heard a happy whooping noise coming from inside; though she doesn't know who it was, Harry suspects Malfoy. Harry encourages Trelawney to tell Dumbledore. On their way, Trelawney mentions the time when she was in Dumbledore's office and made a prophecy (the prophecy about him and Voldemort) and she and Dumbledore were interrupted by Snape. Harry realizes that Snape had overheard the prophecy and he was the one who told Voldemort, which ultimately led to the death of his parents. Harry tells Trelawney to stay there and runs to Dumbledore's office. Dumbledore tells Harry that he thinks he has found another Horcrux and suspects it in in a cave on the seashore where Voldemort had terrorized two children from the orphanage. He wants Harry to come with him. Dumbledore can sense that something has made Harry very upset, and Harry finally confesses that he knows that Snape overheard the prophecy and told Voldemort about it. Harry is mad that Dumbledore still trusts Snape. Dumbledore says that Snape had no idea that Voldemort would attack his parents and that he feels great remorse for what happened. Harry does not believe Dumbledore's assurances and still suspects Snape and Malfoy are up to something. He tells Dumbledore about what Trelawney had overheard, but Dumbledore does not wish to discuss the matter further. He asks Harry if he wants to accompany him in finding the next Horcrux. Harry says yes and Dumbledore makes him agree to do whatever he says. He tells Harry to fetch his Invisibility Cloak and to meet him in the entrance hall. Harry hurries up to his dormitory to collect his cloak. On his way out, he gives Ron and Hermione his Marauder's Map and his leftover Felix Felicis and tells them to keep an eye on Snape and Malfoy and to contact all the D.A. (Dumbledore's Army) members. They start to protest, but Harry says he has no time and suspects something will happen that night and leaves to meet Dumbledore. Harry and Dumbledore walk out towards Hogsmeade and off of school grounds so they can apparate. On their way through Hogsmeade, Madame Rosmerta of the Three Broomsticks pub asks Dumbledore why he is out so late and he says he's on his way to Hog's Head. Harry and Dumbledore apparate to the seashore Voldemort visited as a child.
Harry Potter: The Cheat Sheet [HP on HP]