Because of his ineptitude at handling Voldemort’s supposed return, Cornelius Fudge has no choice but to resign as the head of Ministry of Magic. With Voldemort in power, neither muggles nor wizards are safe.
Professor Severus Snape receives a visit from Draco Malfoy’s mother, Narcissa, and her sister, Bellatrix Lestrange. Narcissa makes Snape swear using the unbreakable vow that he will protect her son while at Hogwarts. The younger Malfoy is on an important mission from Lord Voldemort.
Professor Dumbledore pays a visit to the Dursleys. He’d like Harry to accompany him on a visit to see Professor Slughorn, a retired teacher. Harry helps Dumbledore to convince Slughorn to come back to Hogwarts.
Harry spends the remainder at the summer visiting the Weasleys. While there, he learns Ron’s brother Bill is engaged to be married to Fleur Delacour. The teens receive the results of their O.W.L.s, or Ordinary Wizarding Levels. It’s no surprise to anyone that Hermione receives high marks. Ron and Harry each failed a couple of courses, which means they won’t be able to take their N.E.W.T.S in Potions. This is especially bad news to Harry, who needs this to become an Auror.
In School Again
Harry is surpised to find out Professor Snape is the new Defense against the Dark Arts teacher, while Slughorn will take over in Potions. This is very good news for Harry as Slughorn isn’t as strict, and Harry once again has a chance at being an Auror.
Professor Slughorn loans Harry a textbook once owned by the “Half Blood Prince.” The book is marked, enabling Harry to sail through the class with high marks. As a reward, Slughorn give Harry a small vial of Felix Felicis, a good luck potion.
Dumbledore has special private lessons for Harry. They use the pensieve to view memories of some of the people who were a part of Voldemort’s past, in hopes of gaining information to help them take him down for good.
Dumbledore theorizes that Voldemort was able to come back to being because he created seven horcruxes, or pieces of his soul, before his death. Six are stored elsewhere, while one is inside the Dark Lord himself. This is how he was able to remain in a semi-living state when his death curse rebounded.
If Harry and Dumbledore want Voldemort to die, they will have to destroy all seven horcruxes. Two of them are already gone; Tom Riddle’s diary (see Chamber of Secrets) and a ring belonging to Marvolo Gaunt, a dark wizard.
Professor Snape suspects Harry is doing so well in potions because he found the Half Blood Prince’s old text book. He confronts Harry and demands the book, but Harry gives him Ron’s instead. Professor Snape gives Harry detention at the same time Harry is to help Gryffendor’s Quiddich team win the cup. Ginny Weasley substitutes for Harry, does a tremendous job, and the two begin dating.
Draco Malfoy’s Mission
With his father imprisoned at Azkaban, a reluctant Draco Malfoy takes over his father’s role and becomes a Death Eater. Harry suspects Malfoy is up to no good, and leaves Ron and Hermione with what’s left of his good luck potion, while he and Dumbledore search for Salazar Slytherin’s locket, another Horcrux.
In order to retrieve the Horcrux, Dumbledore must ingest a potion that will leave him very weak. Harry helps Dumbledore return to Hogwarts. As they arrive, they notice the Death Mark in the air.
Malfoy ambushes the duo in the astronomy tower, but he doesn’t know Harry is there as he’s hiding under an invisibility cloak. Sensing what’s about to happen, Dumbledore paralyzes Harry.
Deatheaters arrive and remind Malfoy of his mission, to kill Dumbledore. Malfoy has second thoughts. Snape arrives and Dumbledore pleads with him “Severus….please…” Snape raises his wand and issues the deadly Avada Kedavra curse, killing the beloved wizard and releasing Harry from his paralysis. Snape tells Harry he’s the Half-Blood Prince and flees, taking Malfoy with him.
Harry finds Dumbledore’s body and retrieves the locket promising to destroy it.
After Professor Dumbledore’s funeral, Hogwarts closes down for the remainder of the term, its future uncertain. Though he loves her, Harry breaks up with Ginny, and vows not to return back to school, but to instead search for the remaining Horcruxes, so he can find Snape and destroy Voldemort. He won’t be alone, as Ron and Hermione plan to accompany him. With at least Bill and Fleur’s summer wedding to look forward to, Harry departs for the Dursley’s one last time.