A great treachery is revealed, a great light is snuffed out and the final quest is set up in Harry Potter and the Half-Blood Prince, an emotional and involving installment in the hit-or-miss Harry movie series.

It’s a film of gloom, fear and dread — and we’re not just talking about the teen romance. A great cast is given a little room to breathe and act, something the weakest films in this planned octology have been lacking. The result is a satisfying film, just coherent enough, just engrossing enough to cover for the tedious by-the-book slog of even the best Harry Potter films.s

In year six , Harry is summoned back to Hogwarts School to get to know a new-old teacher, Potions Professor Horace Slughorn, who’s given a Dickensian twinkle by Oscar winner Jim Broadbent. Head master Dumbledore (Michael Gambon) is closing in, he thinks, on the evil Lord Voldemort, whose Death Eaters are brazenly messing with wizard and Muggle alike. Slughorn may be of some use.

But Harry’s task is made trickier by those Harry hormones. If he’s not flirting with waitresses, he’s mooning over the Girl Next Door or watching his best mate Ron (Rupert Grint, funny as always) almost blow it the with out-of-his-league Hermione (plucky Emma Watson).

“The Chosen One” isn’t lacking for confidence. He recklessly hurls himself into tussles. And he learns from the battered copy of Advanced Potions, a book with the name “Half-Blood Prince” scribbled on the inside cover.

Director David Yates, who did the fine BBC version of State of Play before muddling into Potter World with the last film, seems to find more of his footing here. Half-Blood Prince was supposed to come out last winter, but was pulled for bottom-line reasons. The delay shows in the film’s sharper cutting, though, as always, it’s still a half-hour too long. Some of us dream of a fast-paced Potter that skips the obligatory Quidditch match.

The film gives Broadbent several nice scenes, some of them flashbacks. But it also is the first film to let Maggie Smith earn her pay, to not saddle Robbie Coltrane with wizardly exposition and digital critters galore. Gambon brings out Dumbledore’s humanity. And Alan Rickman has more occasions to wrap his voice around the plummy vowels of Severus Snape with a Christopher Walken relish.

“How grand it must beeeee,” Snape purrs, “to be ‘The Chosen One.‘ “

Potter fans can lose themselves in the honey-glow interiors and Christmas-card landscapes, and grin over the butter beer busts and snogging sessions of these typical wizardly teens. And casual fan or hardcore Muggle, Half-Blood Prince lets us see that the end’s within reach, and that with luck and attention to tone and character, this eight-film series (they’re splitting the last book into two movies) will go out with a flash and not a fizzle.

Cast: Daniel Radcliffe, Emma Watson, Michael Gambon, Alan Rickman.

Director: David Yates

Running time: 2 hours 23 minutes

Industry rating: PG for scary images, some violence, language and mild sensuality