As school begins, Snape is unexpectedly announced as the new Defence Against the Dark Arts instructor while Slughorn replaces him as the Potions teacher. Slughorn only requires a minimum "E" grade (Exceeds Expectations) at O.W.L. to take his N.E.W.T.-level Potion classes, making Harry and Ron eligible for the class. Slughorn lends them old Potions textbooks until they buy their own. Harry's copy is marked on the bottom of the back cover, "This book is the Property of the Half-Blood Prince". The talented former owner's notes help Harry excel in the class (as well as learn a few new of his original spells, some even including the dark arts), but the true identity of the "Half-Blood Prince" remains a mystery. As a reward for Harry's performance in Potions, Slughorn gives him a small vial of Felix Felicis, a good luck potion.
Death Eater attacks continue throughout the year and may be linked to events at Hogwarts. On the first Hogsmeade visit, Katie Bell, a Gryffindor student, is seriously injured while carrying a cursed necklace through Hogsmeade, apparently while under the Imperius Curse. In another incident, Ron accidentally drinks poisoned mead intended for Dumbledore. Harry reacts by administering the Bezoar he had submitted previously as an assignment to Slughorn. Hermione is so distraught over this that she and Ron, who were feuding mostly over Ron dating Lavender Brown and her relationship with Viktor Krum, reconcile; Ron soon breaks up with Lavender. Meanwhile, Harry realises his true feelings for Ginny, although she is now dating Dean Thomas.