"Harry Potter and the Half-Blood Prince," the sixth book in J.K. Rowling's Harry Potter series, begins with an overview of the dire state of the wizarding world. Voldemort and his Death Eaters are causing chaos, fear, and death, even influencing the Muggle world.
The British Prime Minister receives a visit from the former Minister for Magic, Cornelius Fudge, who informs him that Voldemort is back. Fudge has been sacked and replaced by Rufus Scrimgeour.
Harry, meanwhile, is whisked away from the Dursleys by Albus Dumbledore. Before heading to the Burrow (home of the Weasley family), Dumbledore takes Harry to the village of Budleigh Babberton to convince retired Hogwarts professor Horace Slughorn to return to teaching. Slughorn taught Potions, and he's known for his habit of "collecting" students who show promise in some way, hoping they will provide him benefits in the future.
Harry learns that he has inherited everything that belonged to Sirius Black, his godfather, who died in the previous book. This includes the house-elf Kreacher, who Harry sends to work at Hogwarts so he won't cause trouble.
At Hogwarts, Harry is surprised to find that his Potions teacher is now Slughorn, and the new Defence Against the Dark Arts teacher is Severus Snape, a position he's long coveted.
As the school year progresses, Dumbledore privately tutors Harry, showing him memories related to Voldemort's past in the Pensieve. These memories reveal crucial aspects of Voldemort's personality, including his obsession with immortality, his disdain for anything less than pure-blood wizards, and the concept of Horcruxes - dark magical objects where Voldemort has hidden pieces of his soul to ensure his survival. One critical memory from Slughorn is missing vital information about Horcruxes, and Dumbledore tasks Harry with retrieving it.
Meanwhile, Harry comes into possession of an old Potions textbook once owned by the "Half-Blood Prince". The book contains helpful notes, spells, and amendments to potion recipes, making Harry excel in the class. But it also contains some dark spells, one of which Harry uses in a confrontation with Draco Malfoy, seriously injuring him.
Harry is convinced Draco is up to something and spends much of the year trying to uncover it. Draco has, in fact, been given a task by Voldemort: to kill Dumbledore. Draco's mother, Narcissa, and aunt Bellatrix go to Snape for help, leading to the Unbreakable Vow - if Draco fails, Snape will do the job.
Harry's personal life is complicated, too. He finally acts on his growing feelings for Ginny Weasley, Ron's younger sister, and they begin dating. Ron, oblivious to Harry's feelings for Ginny, has his own romantic entanglements with Lavender Brown, causing a rift with Hermione, who harbors feelings for Ron.
As the school year nears its end, Harry retrieves the real memory from Slughorn. It reveals a young Voldemort inquiring about Horcruxes, specifically the possibility of making seven. Dumbledore concludes that Voldemort split his soul into seven parts: the diary from "Chamber of Secrets," a ring, the locket of Salazar Slytherin, the cup of Helga Hufflepuff, the diadem of Rowena Ravenclaw, his pet snake Nagini, and the part inside Voldemort himself.
Dumbledore and Harry set off to retrieve a suspected Horcrux, Slytherin's locket, from a cave protected by powerful enchantments. After a harrowing experience where they encounter Inferi (corpses bewitched by a Dark wizard), they secure the locket, but Dumbledore is severely weakened.
Upon their return to the Astronomy Tower at Hogwarts, they find the Dark Mark (Voldemort's sign) in the sky. Dumbledore instructs a very weak Harry to fetch Snape, but before he can leave, Draco Malfoy appears, intending to fulfill his mission to kill Dumbledore. However, Draco hesitates, unable to commit the act.
Then, other Death Eaters, including Fenrir Greyback, storm into the Tower. At this critical moment, Snape arrives and performs the Killing Curse on Dumbledore, sending him over the parapet of the Tower to his death. Harry, paralyzed under his invisibility cloak by a spell from Dumbledore, can only watch in horror.
Once Dumbledore is dead, the spell paralyzing Harry lifts. He pursues Snape, who reveals that he is the Half-Blood Prince. The Death Eaters flee Hogwarts, and the school community is left in shock.
In the aftermath, Harry, Ron, and Hermione make a pact to leave Hogwarts next year, foregoing their final year of school. Instead, they plan to seek out and destroy the remaining Horcruxes, hoping to finally defeat Voldemort.
Harry also opens the locket they risked so much to get, only to find it's a fake. A note inside from someone with the initials "R.A.B." reveals that they have the real Horcrux and intend to destroy it. Harry is determined to find the real locket and the other remaining Horcruxes.
The book ends with the wizarding world mourning Dumbledore's death. Harry, full of resolve, prepares for the daunting task ahead, while the fate of Hogwarts remains uncertain as the fight against Voldemort enters a new, grimmer stage.