Harry Potter and the Half-Blood Prince is the sixth book in J.K. Rowling's Harry Potter series. In this novel, Professor Snape makes an unbreakable vow to protect Harry Potter's rival, Draco Malfoy. Harry and Dumbledore look into memories in an attempt to learn more about Voldemort, and it is unveiled that he has been placing parts of his soul in objects called Horcruxes that must be destroyed if Voldemort is to be defeated.
Severus Snape, a member of the Order of the Phoenix, meets with Narcissa Malfoy, Draco's mother, who expresses concern that her son might not survive a dangerous mission given to him by Lord Voldemort. He makes an Unbreakable Vow with her to protect and assist Draco in his mission.
Dumbledore takes Harry to the temporary home of Horace Slughorn, former Potions teacher at Hogwarts, and persuades Slughorn to return to teach. Harry is taken to the Burrow, where Hermione has already arrived. The next morning they get their Ordinary Wizarding Level (O.W.L.) results, along with lists of school supplies. Later, Ron Weasley and Hermione Granger follow Draco to Dark Arts supplier Borgin and Burkes. Harry is instantly suspicious of Draco, whom he believes to be a Death Eater.
The students return to school, where Dumbledore announces that Snape would be teaching Defence Against the Dark Arts this year, while Slughorn would resume his post as Potions teacher. Harry now excels in Potions, thanks to having received a used Potions textbook that once belonged to someone named "The Half-Blood Prince", who wrote numerous tips and spells in his Potions textbook. The Half-Blood Prince's tips help Harry win a bottle of Felix Felicis, more commonly known as "Liquid Luck", from his lessons. However, Harry's newfound brilliance in potions angers Hermione and causes much tension between the two.
Believing that Harry needs to learn Voldemort's past to gain advantage in a foretold battle, Dumbledore schedules regular meetings with Harry, in which they use Dumbledore's Pensieve to look at memories of those who have had direct contact with Voldemort. Harry learns about Voldemort's family and his evolution into a murderer obsessed not only with power, but with gaining eternal life.
Using his Felix Felicis, Harry eventually succeeds in retrieving one of Slughorn's memories about how he revealed the secrets about splitting one's soul and hiding it in several objects called horcruxes, granting Voldemort a sort of immortality. Dumbledore explains that in order to make Voldemort mortal, all the horcruxes must be destroyed. Two horcruxes, including Riddle's Diary from Harry Potter and the Chamber of Secrets, have already been destroyed but four others remain.
The love-lives of the main characters come into play during the year. Ron and Hermione grow closer together, but after learning from his sister Ginny that Hermione had previously kissed Viktor Krum, Ron shuns her. Harry's attempts to repair things between the two fail, resulting in Ron going out with Lavender Brown to make Hermione jealous. Ron and Hermione later reconcile after Ron is nearly killed in an attempt on Dumbledore's life. As a result of this, Ron and Lavender break up when Lavender sees the two of them walking out of the girl's dormitories together. However, this was only because she hadn't noticed Harry, who had already hurried on ahead. Harry meanwhile falls in love with Ginny, but he is reluctant to enter a relationship with her for most of the year because of his friendship with Ron. He does enter in a relationship with her though, with the consent of Ron.
Later in the year, Harry and Dumbledore journey to a cave to retrieve a horcrux, Slytherin's locket. Dumbledore expertly finds a secret passageway to a hollow, where Harry and Dumbledore take a boat across a dangerous lake. They reach the basin where the locket is hidden underneath a potion. Dumbledore drinks the potion, which severely weakens him, and they fight off Voldemort's Inferi. They take the locket, and return to find that the Dark Mark has been placed over Hogwarts in their absence. Dumbledore uses his magic to freeze Harry in place while Harry remains hidden by his cloak of invisibility. Draco arrives, accompanied by Death Eaters that he helped get inside Hogwarts. Draco disarms Dumbledore of his wand and then threatens to kill him, acting on his mission from Voldemort. Dumbledore tries to stall Draco, who is unable to go through with it, but Snape arrives and kills Dumbledore.
Because of Dumbledore's death, his spell on Harry is broken, and Harry rushes after Snape to avenge Dumbledore. Snape reveals that he is the Half-Blood Prince and escapes with Draco and the other Death Eaters. Later, Harry finds out that the locket is not the real horcrux, containing only a note from someone named "R. A. B.".
After Dumbledore's funeral, Harry breaks up with Ginny, saying it is too dangerous for their relationship to continue. Harry is so devastated by Dumbledore's death that he decides to tell his friends he will not be returning to Hogwarts next year and will instead search out and kill Voldemort by destroying all of the horcruxes. Ron and Hermione vow to join him in destroying Lord Voldemort for good.