Harry Potter and the Half-Blood Prince

Narcissa Malfoy convinces Hogwarts professor Severus Snape to make an Unbreakable Vow to protect her son, Draco, while Draco carries out a mission for Lord Voldemort at the school. Meanwhile, Harry Potter is escorted by Hogwarts headmaster Albus Dumbledore to spend the summer with the Weasley family. On the way, they stop to persuade former Hogwarts professor Horace Slughorn to return to the school to teach Potions. While shopping for school supplies, Harry, Ron Weasley, and Hermione Granger see Draco making enquiries at a shop associated with the Dark Arts. Harry later overhears Draco claiming that he has become a Death Eater.
At Hogwarts, Harry excels at Potions after discovering an old textbook filled with annotations written by the "Half-Blood Prince"; Harry wins a vial of the luck potion Felix Felicis in a class competition. He learns about Voldemort's past by viewing memories in a Pensieve with Dumbledore. Voldemort grew up in a Muggle orphanage, where he used his powers to harm other children. In his later years at Hogwarts, Voldemort questioned Slughorn about a Dark magical object called a Horcrux, which grants immortality. Dumbledore asks Harry to retrieve a memory from Slughorn, as the memory he showed Harry had been altered.
Meanwhile, Gryffindor student Katie Bell is nearly killed. Harry suspects Draco is responsible but lacks proof. He continues trying to expose Draco while fulfilling his duties as captain of the Gryffindor Quidditch team. He also realises that he has feelings for Ron's sister, Ginny. Ron feels excluded from his friend group because he is the only one not invited to an exclusive club run by Slughorn. Hermione invites Ron to the club's Christmas party, but he argues with her after learning about her past relationship with Viktor Krum. Afterwards, Ron begins to perform poorly in Quidditch matches.
Harry tries to repair Ron and Hermione's relationship and restore Ron's confidence, but he fails. Ron begins dating Lavender Brown to spite Hermione, prompting Hermione to lash out at him. At Slughorn's party, Harry overhears Draco refusing Snape's offer to help him with his mission. Harry informs Hermione, Arthur Weasley, and Remus Lupin, but they believe Snape was spying for Dumbledore. Ron and Hermione reconcile after Ron almost dies by poisoning. After multiple failed attempts to obtain the memory, Harry uses Felix Felicis to successfully persuade Slughorn to relinquish it.
The memory reveals that Slughorn told Voldemort how to create a Horcrux: by committing murder, a wizard can split their soul into parts which can be placed in enchanted objects. Dumbledore believes that Voldemort has split his soul into seven parts, each of which is contained in a Horcrux. Dumbledore deduces that two Horcruxes have already been destroyed, but four are still hidden. Meanwhile, Harry begins dating Ginny and duels Draco, severely injuring him with spells taken from the Potions textbook. Harry learns that Snape informed Voldemort of the prophecy concerning him, which led to the murder of Harry's parents. When confronted, Dumbledore defends Snape and claims that Snape regretted his actions.
Dumbledore and Harry encounter several obstacles while retrieving a Horcrux from a remote cave. Back at Hogwarts, a weakened Dumbledore is ambushed by Death Eaters, who have infiltrated the castle with Draco's help. Dumbledore immobilises Harry beneath the invisibility cloak, before Draco disarms Dumbledore. Harry then watches as Snape arrives and kills Dumbledore. The Death Eaters escape and Harry gives chase and subdues several of them, but is ultimately defeated by Snape. Before escaping with Draco, Snape also reveals that he is the Half-Blood Prince to Harry.
Harry learns that the Horcrux he retrieved with Dumbledore is a fake. At Dumbledore's funeral, Harry ends his relationship with Ginny to protect her and decides to find and destroy the Horcruxes with Ron and Hermione.