The use of different directors for “Harry Potter” films was a stroke of genius—it allowed the series to develop, stylistically as well as narratively, as the story deepened and darkened. But the tactic hasn’t made any appreciable difference to the “Twilight” franchise. Bill Condon is a very talented man—“Gods and Monsters” was extraordinary, and “Kinsey” quite good. But he can’t do anything more with this teen-angst, vampire-and-werewolf soap opera than Catherine Hardwicke, Chris Weisz and David Slade did in the three previous installments. The source material is hopeless, that’s all there is to it; and the instruments at the director’s disposal—from the cast down through the crew—are thoroughly mediocre. It’s a lost cause from word one.

“Breaking Dawn” divides the material of a single Stephanie Meyer volume into two sections, and “Part 1” obviously offers the first of them. It’s the same tactic that was successfully employed in the screen adaptation of the last of J.K. Rowling’s “Harry Potter” books, and it worked because there was more than enough action and incident in that case to justify the extended running-time.

Here there isn’t. “Breaking Dawn—Part I” is a very simple piece that begins with the long-anticipated wedding of swooning teen Bella (Kristen Stewart) and her vampire heartthrob Edward (Robert Pattinson). It then proceeds to their honeymoon, during which she unexpectedly gets pregnant—a dangerous thing, since a human is probably too weak a vessel to survive bearing a vampire’s offspring. But Bella—in what must be a nod toward the pro-life crowd—refuses to end the pregnancy. And Jacob (Tyler Lautner), Bella’s werewolf suitor, gets involved to protect her—and, by extension, Edward’s family—against his own pack, who are out to destroy Edward’s child.

It will come as no surprise that in the end Bella survives her ordeal and lives—after all, another installment is due next year. But “Breaking Dawn—Part I” has expired long before then, a victim of laughably bad writing, wooden acting, distinctly un-special effects and sluggish pacing by Condon and editor Virginia Katz, who seem incapable of giving the picture any rhythm or verve. With the exception of one visually muddy werewolf-vampire brouhaha near the close, there’s remarkably little action to speak of, and the supernatural melodrama is played out solemnly, with long pauses as Pattinson whimpers while Bella suffers her labor pains or Lautner smolders angrily over the turn things have taken. The trio continue to look uncomfortable in their roles, like high school students in an under-rehearsed play. You’d think by now they could play these parts in their sleep, and unfortunately at times you suspect they might be. Or maybe it’s just that they’re causing us to snooze off.

But much of that seems Condon’s fault, since he doesn’t manage to secure much from the more experienced members of the cast either. Billy Burke extracts a few smiles from his lines as Bella’s none-too-pleased father, but otherwise the pickings are pretty slim. Michael Sheen, who appears briefly in a tacked-on scene during the final credits to set the stage for the upcoming Part II, for example, just camps it up in a fashion that seems more suited to the “Underworld” franchise he’s also appeared in than this one. (Maybe he just forgot what movie he was making.)

And surely Condon utterly miscalculated in two sequences. One is a dialogue scene between the wolf Jacob and the other members of his pack, in which the human voices are simply superimposed over the badly-rendered CGI animals to risible effect. (Of course, the fact that the dialogue is laughable doesn’t help.) And the other is the prolonged birthing-and-its-aftermath sequence, filled with blood and guts that include internal-organ animation of the CSI variety. It’s all unpleasant to watch, not to mention so turgidly done that it comes across as sheer padding.

Special mention has to be made of the music score, one of the worst in recent memory. The usually imaginative Carter Burwell supplies incredibly sappy orchestral cues, but even worse are the dreadful pop numbers that “music supervisor” Alexandra Patsavas periodically ladles over the action like a gooey sauce. They make the sequences they accompany feel like bad music videos.

None of this will matter to devotees, of course. They’ve proven themselves so addicted to this mixture of cornball romance and supernatural hokum that they’ll devour it even when it’s so ineffectually packaged as it is in this series. But “Breaking Dawn—Part I” is, like its predecessors, all teeth and no bite. Morose, torpid and sometimes just nutty, it’s probably the worst of the series thus far—which given its already dismal quality, is saying a lot.