Narcissa Malfoy, and Bellatrix Lestrange visit Snape’s house. Voldemort has given Draco a mission and they fear for his life. Snape makes an Unbreakable Vow with Narcissa promising to help and protect Draco on this mission.
Dumbledore escorts Harry from the Dursley’s. Grimmauld Place and Kreacher, have been left to Harry in Sirius’ will. They convince Horace Slughorn, a former Hogwarts teacher, to return to the school. Dumbledore will be giving Harry private lessons this term.
The Trio find Draco in Diagon Alley suspiciously walking into a dark arts shop. He threatens the store owner & shows him something on his forearm. Harry thinks Draco is a Death Eater, but no one agrees.
Harry goes back to Hogwarts. Slughorn is the Potions teacher and Snape teaches Defense Against the Dark Arts. Harry finds a used Potions book with notes scribbled in it, which had belonged to the “Half-Blood Prince”. Harry follows the advice of the notes and excels in potions class.
Harry and Dumbledore’s private lessons involve them going into the pensieve and studying old memories of those who knew Tom Riddle before he became Lord Voldemort. The first memory is at the Gaunt house where Marvolo, Morfin and Merope Gaunt live. Marvolo, Voldemort’s grandfather, has a ring and a necklace that are Slytherin heirlooms. Merope, Voldemort’s mother, tricked the muggle, Tom Riddle Sr., into marrying her.
While visiting Hogsmeade, Harry catches Mundungus Fletcher, a member of the Order of the Phoenix, selling Black family heirlooms which now belong to Harry. On the way back, a student almost dies when she touches a cursed necklace. Harry thinks that Malfoy is behind it, but no one agrees.
Harry and Dumbledore visit Dumbledore’s own memory of meeting the 11-year old Tom Riddle in an orphanage. Tom Riddle is disturbed even at such a young age. The also visit Slughorn’s memory where they find a 6th year Tom Riddle, wearing Marvolo’s ring, and several future Death Eaters in Slughorn’s office. Slughorn is trying to hide something because the memory has been tampered with.
Harry tries to get the real memory from Slughorn, but to no avail. Ron gets poisoned accidentally when he drinks meade from Slughorn’s office. Harry suspects Malfoy.
Harry and Dumbledore visit a memory taking place inside the house of Hepzibah Smith who is showing Tom Riddle, now out of Hogwarts and working at the same dark arts shop visited by Draco, a tea cup that belonged to Helga Hufflepuff, founder of Hufflepuff house. They also visit Dumbledore’s own memory of Tom Riddle asking him for a teaching job at Hogwarts. Riddle already goes by the name of Lord Voldemort and is showing signs of magical transformation. Dumbledore denies him the job and verbally frowns open Voldemort’s decisions.
Harry convinces Slughorn to give him the real memory. In it, Tom Riddle asks Slughorn about Horcruxes, which are objects that contain a piece of someone’s soul; keeping them alive when they die. Riddle asks about splitting your soul 7 times, to which Slughorn becomes highly bothered, but says it’s possible. In order to make a horcrux, one has to commit murder. Dumbledore believes Voldemort has made 6 of them.
Harry is forced to hide his potions book in the Room of Requirement near an old tiara when Snape demands that he see the book. Harry learns from a drunken Professor Trelawney that Snape was the one that told Voldemort of the prophecy, causing Voldemort to kill his parents. This increases Harry’s hatred for Snape. Dumbledore, however, reiterates his trust of Snape.
Dumbledore finds a horcrux and takes Harry with him to destroy it. They go to a cave and find the Slytherin locket under dangerous magical protection. They retrieve it and barely escape. Upon returning to Hogwarts, the Dark Mark has been cast over the castle. They fly to the astronomy tower where Dumbledore freezes Harry and is disarmed by Draco. Several Death Eaters arrive with Snape, who kills Dumbledore a la Avada Kedavra. Harry chases Snape through a battle inside the castle involving the Death Eaters, teachers and members of the DA. Snape stuns Harry, tells him that he is the Half-blood Prince and escapes.
Harry looks inside the locket to find a note:
To the Dark Lord, I know I will be dead long before you read this, but I want you to know that it was I who discovered your secret. I have stolen the real horcrux and intend to destroy it as soon as I can. I face death in the hope that when you meet your match, you will be mortal once more. – R.A.B.
What You Need To Know
Voldemort has made horcruxes. Dumbledore suspects he has split his soul into 7 pieces, a perfect number. He believes the horcruxes are as follows:
1) Riddle’s Diary – destroyed by Harry in Chamber of Secrets
2) Marvolo’s Ring – destroyed by Dumbledore the previous summer
3) Slytherin’s Locket – location unknown
4) Hufflepuff’s cup – location unknown
5) Nagini – Voldemort’s snake
6) Something from Gryffindor or Ravenclaw
7) Voldemort himself
What The Movie Doesn’t Tell You
In the film, Harry mopes around hidden at the top of the Astronomy Tower while Dumbledore gets AK’d by Snape. In the book, Dumbledore freezes Harry while under the Invisibility Cloak because he knew Harry would try to intervene.
Harry and Dumbledore’s conversations are not referenced in the film:
Harry has the power of love. Lilly’s sacrifice for Harry gave him a protection that caused Lord Voldemort’s downfall.
Voldemort, being such a powerful wizard, takes no interest in things he doesn’t understand, like love. He does not love and has no compassion. Voldemort feels that these attributes are signs of weakness.
They discuss the prophecy, which is only significant because Voldemort believed it to be true. Setting great store to the prophecy, Voldemort marked Harry as his equal by transferring powers and causing Lilly to sacrifice her life for Harry.