Dont let the PG rating fool you: The dark arts are back with a vengeance in Harry Potter and the Half-Blood Prince, the enthralling sixth chapter in a movie franchise that keeps managing to surprise just when one would expect it to be puttering along on auto-broomstick. Going a few shades blacker than 2007s already funereal Harry Potter and the Order of the Phoenix, this penultimate Potter picture begins as a double-barreled detective story, with Dumbledore (Michael Gambon) searching for clues to the evil Voldemorts apparent invincibility, while Harry (Daniel Radcliffe) ferrets out an assassin lurking among the Hogwarts student body. Mostly, though, the film concerns itself with matters of destiny and the origins of evil, as one present-day Hogwarts pupil attempts to follow in the footsteps of the Dark Lord himself. The movie (superbly directed, like the previous chapter, by British TV vet David Yates) wont make converts out of the series unfaithful, but Id be lying if I didnt say it gave me as much innocent pleasure as anything Ive seen this year. One particular highlight: Hogwarts newest professor, Horace Slughorn, who is played by the great character actor Jim Broadbent as the sort of fusty, avuncular relic who treats students as equals rather than peons, and who, in turn, validates himself through their successes. But Slughorn also carries a private shame, and the closer Harry gets to discovering why that is, the more he finds in his teacher a fellow tragic, tortured soul.