SPOILERS:
Harry discovers his newfound popularity, learns that Stan Shunpike has been arrested, and holds a frustratingly massive Quidditch tryout that includes a run-in with Cormac McLaggen and Katie, Ginny, and Ron making the team. They visit Hagrid and learn that Aragog is dying, Harry confronts Hermione about having confunded McLaggen during the tryout, and Slughorn invites Harry and Hermione to supper, but Harry is spared having to go when Snape refuses to let him out of detention.