Lord Voldemart’s (Ralph Fiennes) Death Eaters kidnap Garrick Olivander (John Hurt) and destroy the Millennium Bridge in London, killing Muggles. Voldemort’s follower Lucius Malfoy (Jason Isaacs) is sent to Azkaban Prison, so he chooses Lucius’s son Draco (Tom Felton) for a secret mission at Hogwarts School. Draco’s mother Narcissa (Helen McCrory) asks Professor Severus Snape (Alan Rickman) to make an unbreakable vow to protect Draco and complete his task if he fails. Since Draco is in Snape’s House of Slytherin, he agrees.
Harry Potter (Daniel Radcliffe) accompanies Headmaster Albus Dumbledore (Michael Gambone) to ask potions professor Horace Slughorn (Jim Broadbent) to return to Hogwarts. Harry meets up with his friends Ron (Rupert Grint) and Hermione (Emma Watson) to visit the joke-shop of Fred and George Weasley (James and Oliver Phelps) and they see Draco entering Knockturn Alley with a gang of Death Eaters, including one Fenrir Greyback (David Legero), who is a werewolf. On the Hogwarts Express, Harry dons his invisibility cloak and hides in the Slytherin carriage. He is petrified by Malfoy but is saved by Luna Lovegood (Evanna Lynch) with her spectre specs.
In a potions textbook he has borrowed, Harry finds notes and spells left by the Half-Blood Prince and impresses Professor Slughorn with his knowledge. Ron becomes Keeper of the Quidditch Team and goes out with Lavender Brown (Jessie Cave). Hermione finds herself upset. Harry spends Christmas with the Weasleys, and Ron’s father Arthur (Mark Williams) mentions the interest of the Malfoys in the vanishing cabinet. The Weasleys’ home is attacked by Bellatrix and Greyback and Harry saves Ginny (Bonnie Wright) before the Order of the Phoenix arrives and routs the Death Eaters.
At Hogwarts, Dumbledore asks Harry to capture a memory of Voldemort from Slughorn. Ron accidentally takes a love-potion meant for Harry and is cured with the help of Slughorn. They celebrate with mead meant for Dumbledore, but Ron is poisoned, and Harry saves his life. Ron murmurs Hermione’s name in his fever and Lavender is devastated. Harry confronts Draco about the poison attack on a fellow student and they duel. Harry seriously injures Malfoy, who is saved by Snape. Ginny and Harry hide the dangerous dark magic book in the Room of Requirement and sneak a kiss.
Harry uses liquid luck potion to force Slughorn to give up his memory of Voldemort for Dumbledore. Harry learns that Voldemort is looking for Horcruxes, in which a wizard might hide his soul for safekeeping. Dumbledore thinks Voldemort divided his soul into six horcruxes and two of them have been destroyed—Tom Riddle’s diary and Marvolo Gaunt’s ring. Harry accompanies Dumbledore to a secret cave, where Dumbledore drinks a potion hiding another Horcrux, Slytherin’s locket.
Dumbledore is weakened but defends them from an Inferi attack and apparates back to Hogwarts, where Bellatrix, Greyback, and other Death Eaters enter through the vanishing cabinet into the Room of Requirement. Harry hides, but Draco disarms Dumbledore. He was sent by Voldemort to kill the headmaster but hesitates. Snape arrives and murders Dumbledore. Snape is the Half-Blood Prince. Students and staff mourn Dumbledore. Harry, Ron and Hermione abandon their final year at Hogwarts to track down and destroy the remaining horcruxes.
The film was directed by David Yates. He described it as a cross between the chills of the Prisoner of Azkaban and the adventure of the Goblet of Fire. Emma Watson almost considered not returning for the film but decided she couldn’t stand to see someone else play Hermione. The lighting and the colour palette of the production were considered important. They were inspired by the paintings of Rembrandt and the film was nominated for Best Cinematography at the Oscars. It was nominated by BAFTA for production design, and Alan Rickman was nominated for Best Supporting Actor.
The harp that put Fluffy to sleep and the King from the giant chess-set from The Philosopher’s Stone are located in the Room of Requirement. The collapsing Millennium Bridge, Harry flirting with the waitress, and the Death Eater attack at the Weasleys’ Burrow were not in the book but were added for the movie. Seven thousand girls auditioned for Lavender Brown, but they went with Emma Watson’s recommendation of Jessie Cave, though she hadn’t auditioned. Later, she complained that she was criticized for gaining weight. Warner Brothers postponed the release date by one year and received death threats.
Stephen King and J.K. Rowling referred to each other’s characters in their books. Bonnie Wright as Ginny Weasley appeared in all the movies but did not get prominent billing until Harry kissed her. The tapestry in the Room of Requirement is from the Hunt of the Unicorn, in the Cloisters Museum in New York. Alan Rickman enjoyed Dumbledore’s fall from a great height because he ended that way in Die Hard. Severus Snape was the Half-Blood Prince because his mother was a Muggle named Eileen Prince. The werewolf who infected Remus Lupin was Fenrir Greyback. A major battle between the Death Eaters and the Order of the Phoenix in the book was excised because it was similar to one in the final movie. Dumbledore asked Snape to kill him because his curse was so painful, and to protect Draco. His fall from the tower is foreshadowed in the Tarot Card. The ending of the movie was given away by Sideshow Bob on the Simpsons.