Instead, the basis of the story continues to be the struggle between a now-teenage Harry (Daniel Radcliffe) and the forces of darkness known as the Death Eaters whose leader, Lord Voldemort, murdered Harry's parents while he was still an infant.

As Harry prepares for another year at the Hogwarts School of Witchcraft and Wizardry, his wise headmaster and mentor Albus Dumbledore (Michael Gambon) asks him to ingratiate himself with incoming potions professor Horace Slughorn (Jim Broadbent), a veteran faculty member returning to Hogwarts after an absence of many years.

Dumbledore believes Slughorn's memories of the youthful Voldemort — whom he taught when the archvillain was a relatively innocent Hogwarts student known as Tom Riddle — may explain Voldemort's embrace of evil and help to defeat him. As seen in flashbacks, Riddle is a potentially good character ultimately led astray by his desire for power.

Harry's principal student rival, Draco Malfoy (Tom Fenton), is driven — by jealousy of Harry's prophesied status as the "Chosen One" — to act as Voldemort's agent within Hogwarts, though a scene in which he breaks down in sobs while alone shows the strain this alliance causes him, and suggests that he, too, could be capable of better things.

Temptation comes Harry's way when he discovers a series of secret notes in his worn copy of the school's potions textbook made by a long-ago student who signed himself the Half-Blood Prince. While these markings originally prove helpful, they turn out to include destructive curses that could draw Harry across the moral divide.

Both Slughorn and Hogwarts' newly installed teacher of Defense Against the Dark Arts, Severus Snape (a scene-stealing Alan Rickman), seem ambiguous, if not downright suspicious. Slughorn is brilliant but overly status-conscious, and none too anxious to reveal his dealings with Riddle, while the magisterial Snape has a sarcastic tone as black as his habitual clothing.

Interwoven with the main plot is the story of the toll adolescent romantic tensions are taking on Harry's long-standing friendship with Hermione (Emma Watson) and Ron (Rupert Grint). There is much talk of "snogging," i.e. kissing, but — a lone double entendre aside — Steve Kloves' script remains appropriately innocent.

Though some details may confuse viewers who have not seen the earlier installments, a gilt-edged supporting cast, including Maggie Smith, Helena Bonham Carter and Robbie Coltrane, together with top-flight special effects, add luster and help prevent this two-and-a-half-hour epic from dragging.

Harry Potter and the Half-Blood Prince will be shown on both Imax and conventional screens.