Harry Potter & the Half-Blood Prince is the sixth movie in the series, and welcomes back the old cast and adds (finally) Jim Broadbent as Horace Slughorn. This is the "Snape kills Dumbledore" movie, by the way.
Harry and company return to Hogwart's, Harry still shaken by Sirius' death the previous year. Dumbledore isn't doing the "act all aloof" thing anymore. Instead, he's very much taken Harry into his confidence. He asks Harry to get close to the new Potions teacher, Slughorn, because Slughorn has a creepy obsession with getting close to the best students for his "Wall of Fame" (which includes Tom Riddle, among others).
Harry does this with the help of a marked-up copy of a potions book he finds, labeled "Property of the Half-Blood Prince". He becomes a potions expert, but not really, he's just following the recipes and advice in the book. Much of the sub-plots here involve Ron getting a girlfriend (who falls for him watching him play Quidditch) and Harry finally hitting it off with Ginny (to her credit, the actress playing Ginny, Bonne Wright, has grown up into a truly impressive young lady and makes the most of her brief scenes).
Finally, Harry talks Slughorn into showing him what he wants - a memory of Tom Riddle asking about "horcruxes," basically an object into which one puts a piece of one's soul. It turns out Riddle made six horcruxes. Dumbledore has already found and destroyed one, and the diary that Harry destroyed back in Chamber of Secrets was another. Now, Harry and Dumbledore set out to find a third, but doing so poisons Dumbledore. When they return, Death-Eaters attack, and Severus Snape murders Dumbledore and reveals himself to be the Half-Blood Prince.
Like all of the movies, a lot was cut. The subplots they've kept mostly revolve around Tom Riddle, and the romantic relationships between the students. The cinematography is amazing (it was nominated for an Oscar for it), and there's a visual theme of ink and like materials spreading through water. We lose some of the impact of Dumbledore's death not seeing his funeral, but the Dark Mark over Hogwart's was a nice touch.
By this point in the series, everyone's established in the roles, and they don't waste any time on exposition of previous plot points, which I'm finding is a lot easier to take watching these movies a few days apart, rather than a few years. I like this movie, though I find that it feels a little thin for some of the cuts and it doesn't transition as smoothly through the year as Azkaban or Order.
My Grade: B+
Rewatch value: Low
Next up: Harry Potter & The Deathly Hallows pt. 1