Sirius Black's death leaves Harry Potter (Daniel Radcliffe) reeling as Lord Voldemort intensifies his grip on both the Wizarding and Muggle realms. His Death Eaters now infiltrate the Muggle world, wreaking havoc under the guise of broad daylight. Within this tumultuous climate, Draco Malfoy ([Tom Felton]) has been tasked with a covert mission, backed by his mother Narcissa and his aunt Bellatrix Lestrange (Helena Bonham Carter), who seek the assistance of Severus Snape (Alan Rickman). Snape secures their trust by presenting himself as a double agent within the Order of the Phoenix. To further seal their pact, he enters an Unbreakable Vow with Narcissa to protect Draco and fulfill his mission should he falter.
As the school year begins, Harry joins Albus Dumbledore (Michael Gambon) on a quest to retrieve the reclusive Horace Slughorn (Jim Broadbent), a former Potions master who has gone into hiding. After persuading Slughorn to return to teach at Hogwarts, Harry reunites with his closest friends: Ron Weasley (Rupert Grint) and Hermione Granger (Emma Watson). Their adventure takes them to Diagon Alley, where they clandestinely follow Draco and Narcissa into the shadowy depths of Knockturn Alley, discovering a meeting of Death Eaters.
Amid escalating fears, Harry suspects Draco has become a Death Eater, a belief met with skepticism from Ron and Hermione. In an effort to gather information, he uses his Invisibility Cloak to sneak into the Slytherin compartment on the Hogwarts Express, only to be discovered by Draco, who petrifies him and leaves him stranded when the train arrives at Hogwarts. With help from Luna Lovegood, Harry counters Draco's spell, eventually making it back to school.
As classes resume, Harry and Ron find themselves needing to borrow Potions textbooks, and Harry stumbles upon a peculiar one annotated by the mysterious "Half-Blood Prince." This book turns out to be a treasure trove of spells and insights, propelling Harry to excel in Slughorn's class. He impresses Slughorn enough to receive a potion known as Liquid Luck, which he later shares with Ron, resulting in an unexpected victory for the Gryffindor Quidditch team, and kicking off a series of romantic entanglements that sees Ron and Lavender Brown (Jessie Cave) become an item, much to Hermione's chagrin.
As Ron grapples with newfound affection, Harry finds his own feelings blossoming for Ginny Weasley (Bonnie Wright). The atmosphere at Hogwarts grows tense when a necklace of dark origin is introduced into their midst, hinting at curses past. Over the Christmas holidays, Harry’s suspicions about Draco intensify, yet they are dismissed by the Order. An attack on the Burrow by Bellatrix and Greyback further pressures Harry as Dumbledore reveals critical memories related to Tom Riddle (Hero Fiennes Tiffin) and the history of Voldemort’s dark magic.
As events spiral, Ron accidentally consumes a love potion destined for Harry, leading to a series of complications that end in Ron being poisoned. After a close call, Harry confronts Draco, leading to an intense confrontation where he draws from the Half-Blood Prince’s book to inflict harm, only for Snape to intervene. This encounter cements Snape's ties to dark magic, as he reveals himself as the elusive Half-Blood Prince.
In a climactic episode, Harry uses a Liquid Luck potion to persuade Slughorn into confronting his memories about Horcruxes, revealing Voldemort's quest for immortality through the very essence of splitting one's soul. On a dangerous journey to recover one of these hidden pieces, Harry and Dumbledore face off against Inferi while navigating treacherous waters.
Ultimately, the tone shifts dramatically when Dumbledore, having been weakened, confronts Draco upon their return to Hogwarts. But the young Malfoy’s struggle with his mission results in Severus Snape striking the deadly blow, leaving Harry in a haze of grief as he returns to find the castle in mourning. In the aftermath, Harry shares with Ron and Hermione that the Horcrux they thought they located was merely a decoy, pushing the trio toward a new goal: to seek and destroy Voldemort’s remaining Horcruxes rather than return for their final year at Hogwarts.
