Harry Potter and the Half-Blood Prince

Plot :

Further information: Plot of the novel

   Harry is shown bleeding in front of the Ministry of Magic, which took place after the second to last scene of the previous film, Harry Potter and the Order of the Phoenix.Lord Voldemort is tightening his grip on both the Wizarding and Muggle worlds and has chosen Draco Malfoy to carry out a secret mission. Severus Snape accepts Bellatrix Lestrange's challenge to make an Unbreakable Vow with Draco's mother, Narcissa, to protect Draco and fulfill the assignment if he fails.

   Harry accompanies Albus Dumbledore to visit former potions professor Horace Slughorn, who has gone into hiding. Slughorn agrees to return to teach at Hogwarts as Dumbledore tells Harry that Slughorn's return to Hogwarts is crucial.

   Leaving Fred and George's new shop, Harry, Ron and Hermione notice Draco and Narcissa associating with Death Eaters in Borgin and Burkes. Harry believes Voldemort has made Draco a Death Eater, but Ron and Hermione are skeptical. On the Hogwarts Express, Harry spies on Draco, who hints to his friends that he does not plan to return to Hogwarts next year. Draco discovers Harry's presence and immobilizes him, but Harry is rescued by Luna.

   At Hogwarts, Harry and Ron are admitted to Slughorn's potions class at the last minute and borrow the needed textbooks. The previous owner of Harry's copy, the "Half-Blood Prince", has annotated the book with additional instructions that allow Harry to excel in class. After making many spectacular saves as Keeper on the Gryffindor Quidditch team, Ron becomes a Gryffindor hero and forms a relationship with Lavender Brown, leaving Hermione heartbroken. Harry discovers Hermione sobbing in the Astronomy Tower and Harry reveals he has feelings for Ron's sister, Ginny Weasley.

   Harry spends Christmas with the Weasleys and becomes closer to Ginny. They almost share a kiss on Christmas Eve but Bellatrix and Fenrir Greyback, intent on capturing Harry, attack and burn the Burrow. Outnumbered, they abandon their attack as Order members Nymphadora Tonks and Remus Lupin, together with Harry and the Weasleys, watch their home burn.

   Suspecting that Draco is responsible for two attempts on Dumbledore's life, one of which nearly kills Ron (who, in his recovering state, admits that he loves Hermione), Harry confronts him and severely wounds Draco with the Sectumsempra curse from the Half-Blood Prince's book. Harry, shocked that the curse causes Draco's chest to be sliced open, retreats as Severus Snape enters and heals Draco. Fearing the book may be filled with more Dark Magic, Ginny and Harry hide the book in the Room of Requirement and share their first kiss.

   Dumbledore shows Harry memories of a young Tom Riddle and reveals that Slughorn retains a memory critical to Voldemort's defeat. Harry retrieves the memory, learning that Voldemort wanted information for creating seven Horcruxes. The Horcrux safeguards a portion of the creator's soul, granting him immortality unless the Horcrux is destroyed. Two of Voldemort's Horcruxes have already been destroyed: Tom Riddle's diary and Marvolo Gaunt's ring. Touching the ring, Harry has a vision of Voldemort, which is noticed by Dumbledore. After discovering the possible location of another Horcrux, Dumbledore requests Harry's help to retrieve it. They travel to a far-away cave where Harry is forced to make Dumbledore drink a mind-altering potion that hides the Horcrux, a locket. A weakened Dumbledore defends them from a horde of Inferi and apparates himself and Harry back to the Astronomy Tower at Hogwarts. In the meantime, Bellatrix, Greyback and several other Death Eaters enter the castle with Draco's help.

   Upon arriving at the tower, Dumbledore tells Harry to fetch Snape but as footsteps approach, he orders Harry to hide. Draco reveals that he has been chosen by Voldemort to kill Dumbledore, but he is unable to follow through. Snape arrives and casts the Avada Kedavra curse, killing Dumbledore. As Bellatrix casts the Dark Mark, Snape, Draco and the Death Eaters escape from the castle. Bellatrix destroys the Great Hall and sets fire to Hagrid's hut as Harry attempts to stop them. Snape overpowers Harry and reveals that he is the Half-Blood Prince before escaping. Harry returns to the school to find the staff and students mourning Dumbledore. Together they destroy the Dark Mark to honour Dumbledore.

   Harry later reveals to Ron and Hermione that the locket Horcrux was a fake. The locket contains a message from an "R.A.B." to Voldemort stating that he has stolen the real Horcrux with the intent of destroying it. His dying wish is that when Voldemort meets his match he will be mortal once more. Rather than return for their final year at Hogwarts, Harry, Ron and Hermione vow to seek out who R.A.B. was and to find the remaining Horcruxes.

From : Wikipedia