At his home in Spinner's End, Severus Snape receives a visit from Draco Malfoy's mother Narcissa and her annoyed sister, Bellatrix Lestrange. Narcissa swears Snape to an Unbreakable Vow, ensuring he will protect Draco during his task assigned by the Dark Lord and, should her son fail, complete his mission for the Dark Lord.
Albus Dumbledore arrives at the Dursleys' and asks Harry Potter to accompany him to see retired professor Horace Slughorn. With unwitting help from Harry, Dumbledore persuades Slughorn to resume his old teaching post at Hogwarts. Harry then spends the remaining summer months at The Burrow with the Weasleys and Hermione, who is also visiting. To his family's dismay (mainly Ginny, Mrs Weasley, and Fred & George), Bill Weasley has become engaged to Fleur Delacour, the Beauxbatons champion in the Triwizard Tournament. During this summer Harry spends time with Ginny and experiences the stirring of feelings for her. Harry, Ron, and Hermione receive their O.W.L. results. Hermione receives high marks in all her subjects, but Ron and Harry fail Divination and History of Magic. They are also unable to take N.E.W.T.-level Potions because Snape only accepts "O" (Outstanding) grades. The course is a requirement to Harry's ambition of becoming an Auror. While in Diagon Alley buying supplies for their upcoming year at Hogwarts, Harry, Ron and Hermione spot Draco Malfoy sneaking off into Knockturn Alley, where they follow him and spy on him. They see him enter Borgin and Burkes, a dark magic shop, and witness him harassing Mr Borgin about repairing one item and keeping another safe for him.
At Hogwarts
As school begins, Snape is unexpectedly announced as the new Defence Against the Dark Arts instructor, while Slughorn takes his place as the Potions teacher. Since Slughorn only requires a minimum "E" grade (Exceeds Expectations) at O.W.L. to take his N.E.W.T.-level Potion classes, Harry and Ron are now able to sign up. Not having expected this, they haven't bought any teaching material beforehand, and so Slughorn lends them old Potions textbooks. Harry's copy is marked as the property of "The Half-Blood Prince." The talented former owner's handwritten notes help Harry excel in the class. As a reward, Slughorn gives him a small vial of Felix Felicis, a good luck potion.
As the year progresses, more Death Eater attacks occur which may be linked to events happening at Hogwarts. On the first Hogsmeade visit, Katie Bell, a Gryffindor student, is seriously injured when she is forced to carry a cursed necklace while under the Imperius Curse. Harry suspects Malfoy or another Death Eater is involved. In another incident, Ron is accidentally poisoned when he drinks mead that was intended for Dumbledore ? Harry's quick thinking saves his life when he forces a bezoar, a poison antidote, into his mouth. Prior to this Ron and Hermione had engaged in a vicious feud wherein Ron had dated Lavender Brown mostly to annoy and anger Hermione for having kissed Viktor Krum, and Hermione had retaliated by using Cormac McLaggen as her date for Slughorn's Christmas Party, which backfired so horribly on her that even Harry chastised her for her behaviour. Ron and Hermione's continual bickering had caused Harry to fear they would never reconcile. However, Hermione is so upset and distraught over Ron's near poisoning that she reconciles with him, and Ron eventually breaks up with Lavender. In the meantime, Harry discovers the real nature of his feelings for Ginny, now the Quidditch team's Chaser, when he jealously observes her kissing her boyfriend, Dean Thomas. However, Harry hesitates to do anything because of Ginny's relationship with Dean and a fear that Ron would not react well to Harry dating his only sister.