Harry Potter and the Half-Blood Prince

Lord Voldemort tightens his grip on the wizarding and Muggle worlds: his Death Eaters kidnap wandmaker Ollivander and destroy the Millennium Bridge. With Lucius Malfoy incarcerated in Azkaban, Voldemort chooses his son, Draco Malfoy, to carry out a secret mission at Hogwarts. Draco's mother, Narcissa, and aunt Bellatrix Lestrange seek out Severus Snape, who claims he is a spy in the Order of the Phoenix. Snape makes an Unbreakable Vow with Narcissa to protect Draco and fulfill his task.
Harry Potter accompanies Albus Dumbledore to persuade former Potions professor Horace Slughorn to return to Hogwarts. At the Burrow, Harry reunites with his best friends, Ron and Hermione. In Diagon Alley, they see Draco and Narcissa, and follow them into Knockturn Alley. The pair meet with Death Eaters, including the werewolf, Fenrir Greyback. When Harry believes Draco is now a Death Eater, Ron and Hermione are skeptical.
At Hogwarts, Harry discovers that his used Potions textbook is filled with notes and spells, added by the Half-Blood Prince. Using it, Harry excels in class, impressing Slughorn, who awards him a luck potion. Ron makes the Gryffindor Quidditch team as Keeper and begins dating Lavender Brown, upsetting Hermione, who harbours feelings for him. Harry consoles Hermione while acknowledging his own feelings for Ron's younger sister Ginny Weasley. Harry spends Christmas with the Weasleys. His suspicions about Draco are dismissed by the Order, but Arthur Weasley reveals that the Malfoys may be interested in a Vanishing Cabinet. Bellatrix and Greyback attack the Burrow in which Bellatrix boasts about killing Sirius Black.
At Hogwarts, Dumbledore asks Harry to retrieve Slughorn's memory of a young Voldemort; Slughorn has resisted giving an accurate memory. After Ron accidentally ingests a love potion intended for Harry, Harry takes him to Slughorn for a cure. After curing Ron, Slughorn offers Harry and Ron mead. Ron is poisoned after drinking it, and while recovering in the infirmary, he murmurs Hermione's name, causing Lavender to end their relationship. Harry confronts Draco about the poisoned mead, and also a cursed necklace that nearly killed Katie Bell. Harry and Draco duel, and Harry uses one of the Half-Blood Prince's curses that severely injures Malfoy, and he is only saved by Snape. Fearing the book contains Dark Magic, Ginny persuades Harry to hide it in the Room of Requirement, where they share their first kiss.
Harry uses his luck potion to convince the reluctant Slughorn to surrender the memory Dumbledore needs. Viewing it in the Pensieve, Dumbledore and Harry learn Voldemort sought information about Horcruxes, magical objects containing pieces of a wizard's soul for immortality. Dumbledore believes Voldemort divided his soul into six Horcruxes, two of which have been destroyed: Tom Riddle's diary and Marvolo Gaunt's ring. They travel to a cave where Harry aids Dumbledore in drinking a potion that hides another Horcrux: Slytherin's locket.
A weakened Dumbledore apparates them back to Hogwarts, where Bellatrix, Greyback, and the other Death Eaters have entered through the Vanishing Cabinet in the Room of Requirement that Draco has secretly connected to Knockturn Alley. As Harry hides, Draco appears and disarms Dumbledore, revealing that Voldemort chose him to kill Dumbledore. Draco hesitates; Snape arrives and kills Dumbledore with the Killing Curse. As the Death Eaters escape, Harry attacks Snape, and he reveals to Harry that he is the Half-Blood Prince.
As Hogwarts mourns Dumbledore's death, Harry tells Ron and Hermione that the locket is fake and contains a message from "R.A.B.", who stole the real Horcrux and intended to destroy it. The trio agrees to abandon their final year at Hogwarts, vowing to find and destroy Voldemort's remaining Horcruxes.