After his fifth year at Hogwarts, Harry has spent two weeks mourning the loss of his godfather, Sirius Black. As Albus Dumbledore escorts him to the Weasley home, they visit the retired Hogwarts professor Horace Slughorn, who agrees to resume teaching. Meanwhile, Bellatrix Lestrange and her sister Narcissa Malfoy convince Severus Snape to make an Unbreakable Vow to protect Narcissa's son Draco at Hogwarts. While out shopping for school supplies, Harry, Ron Weasley, and Hermione Granger observe Draco making inquiries at Borgin and Burkes, a shop known for its connection to the Dark Arts. At Hogwarts, the students learn that Slughorn will be teaching Potions, while Snape will be taking over Defence Against the Dark Arts. For Slughorn's first class, Harry and Ron borrow a pair of old textbooks. Harry's textbook previously belonged to someone known as "The Half-Blood Prince", and it contains many helpful tips. Following the instructions of the Prince, Harry becomes an expert potion brewer. He rises to the top of the class and wins a vial of the luck potion Felix Felicis.