Paralyzed under the Invisibility Cloak, Harry is powerless. Suddenly, his cloak flies off and Tonks appears, helping him up. Together, they hurry toward Hogwarts. As Harry enters the Great Hall, students turn and stare. Dumbledore introduces Professor Slughorn as the new Professor of Potions and explains that Snape will now be teaching Defense Against the Dark Arts. Harry, Ron, and Hermione are shocked. Dumbledore has never before trusted Snape with such an important position. Ron points out that the job is jinxed, as no one has ever lasted more than a year as a Defense Against the Dark Arts instructor. As they leave the Great Hall and return to their dormitories, Harry tells Ron what he overheard in the Slytherin compartment. Ron remains skeptical about Draco’s relationship to Voldemort. They run into Hagrid, who tells the boys he’ll see them at Care of Magical Creatures the next morning. Neither Ron nor Harry nor Hermione has signed up for the class. In the past, the three classmates only endured Care of Magical Creatures out of devotion to Hagrid, and they wonder what he will think when he finds out his three best students are no longer interested.
The next morning, Harry fills Hermione on what he overheard in the Slytherin compartment. Like Ron, Hermione is doubtful about Draco’s relationship to Voldemort. Hermione is more concerned about what Hagrid will think when he realizes that none of them have signed up for his class. Before classes begin, Heads of House must sort out the students’ schedules, confirming that each student has received the necessary O.W.L. grade to continue on with their chosen N.E.W.T.s. When it is Harry’s turn, Professor McGonagall asks why he is not continuing with Potions if it is still his desire to one day be an Auror. Harry explains that he did not think his O.W.L. score was acceptable, but McGonagall tells him Slughorn has lower requirements than Snape, and Harry can continue. Harry tells her that he did not buy any of the proper books or ingredients, but McGonagall insists that he can easily borrow them. Moments later, Ron is cleared for the same set of subjects: Charms, Defense Against the Dark Arts, Herbology, Transfiguration, and Potions.
In Defense Against the Dark Arts, students begin work on nonverbal spells. When Harry mouths off to Snape, he receives a detention for Saturday night. Later, Harry receives a scroll from Dumbledore, announcing that their first private lesson will be Saturday night. Delighted that he has a valid excuse to skip Snape’s detention, Harry heads off to Slughorn’s Potions class. Harry and Ron ask Slughorn to borrow two Potions textbooks. Slughorn complies, digging two used books out of a cupboard. Slughorn tells the class that the creator of the best potion will win a small vial of Felix Felicis, an extraordinarily lucky potion. Harry opens his used textbook to the appropriate page, but he finds it difficult to read because the previous owner has scribbled notes all over the pages. Finally, Harry opts to follow the alternate instructions written in the margins, and his potion turns out perfectly. Slughorn is delighted and awards Harry the vial of Felix Felicis, but Hermione is outraged and accuses Harry of cheating. Harry notices some writing along the bottom of the back cover. It reads: “This Book is the Property of the Half-Blood Prince.”
Harry’s fascination with Draco continues to grow in this section. Harry is so convinced of Draco Malfoy’s guilt that he can think of little else. Already, he has twice used his Invisibility Cloak to follow Draco and spy on Draco when he thinks he’s alone. These episodes reflect Harry’s bravery but also demonstrate his inability or unwillingness to think ahead. While Harry does overhear some incriminatory talk between Draco and the other Slytherins in his cabin, Draco obviously suspects Harry’s presence and says nothing that could not be excused as showing off to his friends. Likewise, when Harry, Ron, and Hermione listen in to Draco’s conversation with Borgin, they are not ready to deal with what they hear. Instead, Hermione rushes into the shop prematurely and without a suitable plan and ends up raising Borgin’s suspicions and getting herself kicked out of the shop. Clearly, Harry’s use of his Invisibility Cloak to follow Draco around has not been without its hardships and complications, most of which have been caused by a lack of foresight. As Dumbledore explains in Book V, Harry’s education must come slowly and unfold over time. Harry, however, is more interested in knowing everything as quickly as possible and often gets himself into unfortunate predicaments as a result.
When the train arrives at Hogwarts, Harry’s cover is compromised, Draco freezes him, and he gets himself into an impossible situation, unable to protect himself from attack or even leave the train. When Tonks suddenly appears to help Harry, it becomes clear that Dumbledore and the Order are taking special care to protect Harry this year. Tonks tells Harry that she didn’t see him leave the train and realized he might be hiding. She came on board to make sure he was safe, but her timing is suspiciously perfect. When Harry asks what she was doing in Hogsmeade, Tonks tells him she has been stationed there, but it seems far more likely that she’s actually been assigned to watch over Harry and make sure he remains safe. Harry does not seem to understand Tonks’s role in his wellbeing and is simply relieved to have been rescued and allowed to return to school.
Harry has never been an especially strong Potions student, and with Snape as his instructor he always found Potions confounding and useless. Still, Slughorn continually reminds Harry how his mother, Lily, excelled at Potions, and Harry has figured out that he can use the Half-Blood Prince’s textbook to emulate her success. Hermione is skeptical of and annoyed by the Prince’s book, mostly because she is such a strong student and finds Harry’s use of the Prince’s notes to be unfair and unethical. When Harry wins the vial of Felix Felicis, Hermione is angry that her role in Harry’s life is being threatened. Typically, Hermione is the only one of the three friends who can figure out their schoolwork, make the correct potions, write the acceptable essays, and answer the hard questions. When Harry finds a way to bypass Hermione’s help, she feels hurt and betrayed. She believes that Harry is taking an unnecessary shortcut and that the Prince’s advice may be either dangerous or incorrect. She is envious and troubled by Harry’s newfound study help and cannot seem to understand why he refuses to give the book back.