In the wizarding world, dark forces are at play as Lord Voldemort's influence spreads, both in the magical and Muggle communities. Against this backdrop, the protagonist returns to Hogwarts for his sixth year, along with his friends. The school is no longer the safe haven it once was, with danger lurking inside as much as outside its walls. The headmaster imparts a crucial task to the protagonist; to uncover the secret of Voldemort's immortality – the key to his downfall. A mysteriously annotated schoolbook belonging to the "Half-Blood Prince" falls into the protagonist's hands, endowing him with insights and spells that aid him in his quest.

Amidst growing fears and tightened securities at Hogwarts, the protagonist discovers an old potion book marked by its previous owner, the Half-Blood Prince, which transforms his Potions class experience, earning him newfound acclaim. Meanwhile, emotional entanglements complicate life further as romantic relationships develop among the protagonist’s closest friends, providing a counterpoint to the growing gloom. The school's social dynamics are upended, and trust becomes a scarce commodity as the true allegiances of various characters are called into question. While navigating these turbulent waters, the protagonist draws nearer to the dark past of his nemesis, unveiling key mysteries that were hidden in plain sight.