Ominous clouds gather in scene after scene of "Harry Potter and the Half-Blood Prince." Some are formed in rivulets of inky liquid memories, swirling in a basin to conjure recollections from the faculty of Hogwarts School of Witchcraft and Wizardry. Baleful gray thunderheads mass over the school, with death's heads eyelessly staring at Lord Voldemort's enemies on the campus. Death Eaters spew noxious vapor trails in an electrifying attack on London, scattering terrified Muggles and tearing apart the Millennium Bridge.

Unlike most film series, the Potter movies haven't weakened along the way. The films have done justice to J.K. Rowling's complicated mythology, speaking confidently to the fan base while remaining accessible to the general public. In that, they surpass the "Star Wars" prequels, the "Matrix" trilogy or the "Narnia" films. Their only serious competition in the world of fantasy series would be Peter Jackson's "Lord of the Rings" trio, and Tolkien's text was thousands of pages shorter.

The latest installment of the Potter series deals in grave matters. Harry undertakes a dangerous counterintelligence mission, and there are moments of profound sadness as Someone Important dies in the struggle against the dark lord. Yet this is also a delightfully comic episode, with love potions, romantic rivalries and runaway hormones befuddling Harry, now 17 and in his sixth year at Hogwarts.

Magnificently bearded Prof. Dumbledore (Michael Gambon) greets pink-cheeked Harry with, "you need a shave, my friend." The films are growing more mature along with their hero. A sure balance of powerful drama and giggly teen humor marks this as one of the best Potters yet, a cauldron full of magic.

As Harry (Daniel Radcliffe), Hermione Granger (Emma Watson) and Ron Weasley (Rupert Grint) enter their sixth year as trainee wizards, Rowling cranks the plot into high gear. Following a devastating strike on Diagon Alley, Voldemort's minions are massing for an all-out assault. Dumbledore requires Harry's help in delving into the past to locate seven magical keys that can stop the attack. The usual villains, bully-boy Draco Malfoy (Tom Fenton) and Prof. Snape (Alan Rickman) combine their efforts to give Harry serious trouble -- Malfoy with physical and magic attacks, Snape with deliciously nasty sneers and lip-curling line readings.

An ambiguous new character in the proceedings is Horace Slughorn (Jim Broadbent), the retired Potions master who taught Tom Riddle, the disturbed student who became Lord Voldemort. Harry, now acting as Dumbledore's adult associate rather than his youthful protégé, is assigned to learn from the secretive, fearful Slughorn information about Riddle that can be used to defeat Voldemort. Broadbent, whose character is in hiding from the creepy Death Eaters, makes a brilliant entrance disguised as an overstuffed chair.

Director David Yates trims Rowling's doorstop of a book to its essentials, goosing the action along briskly. Character growth is deftly sketched. Malfoy begins the story as a stock thug, giving Harry a dreadful beating, but he ends the film anguished over the choices he must face. Flashbacks to the cold-eyed Tom Riddle at 11 and 16 (compellingly played by Ralph Fiennes' nephew, Hero Fiennes-Tiffin, and Frank Dillane) provide an ominous look at the villain's twisted roots.

As the love-bedazzled Ron, Grint delivers the film's standout comic performance. Watson's Hermione huffs and scowls in romantic frustration as her two childhood pals steadfastly ignore her almost-adult charms.

Cinematographer Bruno Delbonnel shoots the supernatural world with murky beauty. Wizards' wands are repeatedly shown with their lighted tips straining to dispel encroaching darkness. At the end, scores of wands are lifted together, their massed power pushing back the gloom. It's a promising glimpse at the two-part finale of the series, just over the horizon.

Colin Covert • 612-673-7186