The alchemy that makes a hit musical is as weird and inexplicable as a witch's spell - as the touring "Wicked" demonstrates yet again in the Broadway blockbuster's fourth visit to Houston.

Based on Gregory Maguire's novel telling the back story of the good and bad witches of Oz, "Wicked" soars despite unevenness in Winnie Holzman's book and Stephen Schwartz's score - both with fine moments, yet overall, more serviceable than inspired. "Wicked" lands a phenomenal punch anyway, because the premise is so irresistible and every other key element of the show, from direction to design to performance, is just about ideal.

As always, the first thing you notice is Eugene Lee's elaborate scenic design: a proscenium of gadgets and gizmos, framing the show curtain depicting the map of Oz, topped by the Time Dragon, a huge puppet whose eyes glow red as it breathes fire and flaps its enormous wings. That's the tip-off that "Wicked" is the real deal, a grand-scale, fully appointed Broadway musical - a rare breed these days, whether in New York or on the road.

Once the curtain rises, the rest of Lee's settings, Susan Hilferty's wildly imaginative costumes and Kenneth Posner's magical lighting combine to conjure an extravagant vision, fantasyland with quirky twists - after all, Oz always had a freakish streak. Joe Mantello's fluid and inventive direction melds all the design elements and the company into vivid stage pictures. With eclectic choreographic embellishment by Wayne Cilento, Mantello generates narrative momentum, with increasing attention to the more dramatic aspects as it advances.

The heart of the show is the unlikely friendship between soulful, green-skinned outcast Elphaba and irrepressibly perky and opportunistic Glinda, who meet as college roommates and become rivals for the same man, irresponsible Prince Fiyero. Holzman's book is best portraying the evolving relationship of the two heroines, but has more variable results with the show's political dimension, as Elphaba tries to combat the fraudulent Wizard, who lies to the public and instigates persecution of animals (heretofore the equal of humans in Oz.) The script at times turns glibly jokey, yet scenes revealing origins of familiar "Wizard of Oz" characters prove reliable fun.

Schwartz's score boasts standout numbers, especially Glinda's character number "Popular" (with about 10 times more going on its lyrics than most of the other songs) and Elphaba's best ballad, the quiet, rueful "I'm Not That Girl," as well as Fiyero's "Dancing Through Life" and Glinda's "Couldn't Be Happier" section of the Act 2 opening. Yet too much of the score relies on flat declamations set to musical bombast ("No One Mourns the Wicked," "No Good Deed Goes Unpunished") or settles for generic "American Idol" pop sound.

This cast is solid, with not quite the powerhouse vocal virtuosity of some past teams - yet with particularly strong acting by the two leads, bringing greater conviction and depth to the more dramatic or poignant scenes.

Jennifer DiNoia commands the stage as the somber, crusading, increasingly angry Elphaba. She gets the show's most flamboyant numbers and handles them with the requisite force and range, yet she's particularly good in quieter moments, with a shade more intimacy and grace there than some past interpreters.

Hayley Podschun initially seems a bit uneasy negotiating Glinda's tricky shifts from legit soprano to musical comedy "chest" voice, but she sings with increasing command and control as the show advances. She shows superb comic skills in Glinda's giddy posing and flouncing, yet finds new depths in later poignant moments.

David Nathan Perlow plays callow Fiyero with charm and sings agreeably. Kathy Fitzgerald makes a properly imperious, then sinister Madame Morrible. Walker Jones' Wizard is a genial albeit low-key charlatan. Jenny Fellner and Alex Wyse do nicely with the subplot of lovelorn Nessarose and Boq, her ill-fated swain.

Perhaps inevitably, "Wicked" impresses much as before. You can't keep a good, or wicked, witch down.