After his exposure to the wizarding world, Lord Voldemort is tightening his grip on both the wizarding and Muggle worlds, kidnapping Ollivander from Diagon Alley and destroying the Millenium Bridge. After the Malfoy family is disgraced when Lucius is apprehended and sent to Azkaban for his involvement with the Death Eaters, Voldemort chooses Draco to carry out a secret mission at Hogwarts. Bellatrix Lestrange and Narcissa Malfoy, Draco’s mother, seek help from Severus Snape who claims to have been acting as a mole within the Order of the Phoenix all along and makes an Unbreakable Vow with Narcissa to protect Draco and fulfill the assignment if he fails.
Harry accompanies Albus Dumbledore at a Tube station to visit former Potions professor Horace Slughorn, who has gone into hiding but agrees to return to teach at Hogwarts. Dumbledore then takes Harry to the Burrow, where Harry reunites with his best friends Ron Weasley and Hermione Granger. When the three go visit the Weasley brothers’ new joke shop at Diagon Alley, they see Draco entering Knockturn Alley with a group of Death Eaters, including Fenrir Greyback. Harry believes Voldemort has made Draco a Death Eater, but Ron and Hermione are skeptical. On the Hogwarts Express, Harry hides in the Slytherin carriage using his Invisibility Cloak, but is caught by Malfoy and cursed, although he is saved by Luna Lovegood. At Hogwarts, Harry and Ron are forced to borrow textbooks for Slughorn’s Potions class, and Harry is stuck with a copy that turns out to be filled with helpful notes, instructions, and spells left by the book’s previous owner, the “Half-Blood Prince”. Using the book, Harry excels in the class and impresses Slughorn, winning a vial of Felix Felicis potion, a brew that allows the consumer perfect luck. Ron becomes Keeper of the Gryffindor Quidditch team and forms a romantic relationship with Lavender Brown, upsetting Hermione. Harry consoles Hermione, revealing that he now has feelings for Ron’s younger sister, Ginny Weasley.
Harry spends the Christmas holidays with the Weasleys. On Christmas Eve, Harry discusses his suspicions about Draco to the Order of the Phoenix. Although he is immediately dismissed by Remus Lupin, Arthur Weasley claims that the Malfoys may have been interested in a Vanishing Cabinet. Later, Bellatrix and Greyback raid and set fire to the Burrow. Back at school, Ron is accidentally given a love potion intended for Harry. Although Slughorn cures him, Ron is nearly killed when he drinks poisoned mead, a drink originally intended for Dumbledore. While recovering, Ron murmurs Hermione’s name, causing Lavender to end their relationship. Harry confronts Draco and severely injures him with a Sectumsempra curse taken from the textbook of the Half-Blood Prince. Snape enters and quickly heals Draco’s wound. Fearing the book may be filled with more Dark Magic, Ginny and Harry hide it in the Room of Requirement and share their first kiss.