”We’ve got to get home and start reading,” said one of the teenage girls tearing out the door of the small-town Borders where I bought my copy of Breaking Dawn a few minutes after midnight on Aug. 2. Ardent fans of Stephenie Meyer’s blockbuster series about perpetually swooning Bella Swan, her vampire sweetheart, and the various werewolves and bloodsucking freaks who populate her world aren’t waiting for a cool critical appraisal of this final installment to decide whether it’s worth their time. You succumb to Meyer’s novels as you do to a powerful, slightly ridiculous dream. Or you don’t.

Here’s a third possibility: You whip through Twilight, New Moon, and Eclipse, then abruptly lose all patience with the franchise midway through Breaking Dawn, when Meyer takes her supernatural love story several bizarre steps too far.

Dawn begins days before 18-year-old Bella’s wedding to Edward Cullen, whom she has worshipped since she first arrived in the drizzly town of Forks, Wash., three volumes and 1,700 pages ago. Back then she was the new kid in town, insecure and crushing madly on the cutest boy in school, who just happened to be a vampire. (He’s part of a polite coven who drink animal blood.) Now, after countless misunderstandings and estrangements, after outsmarting evil fiends and consorting with cuddly werewolves, Bella finally has what she has always wanted: Edward. ”He had the most beautiful soul, more beautiful than his brilliant mind or his incomparable face or his glorious body,” Bella coos. Yes, you need to plow through acres of this kind of goo in all Meyer’s novels, but they move so quickly you hardly notice.

The couple has made a pact. Edward will transform Bella into a vampire — a fate she has pleaded for — but only after they enjoy a ”real” honeymoon, complete with vampire-on-human sex. So, off to a tropical island the newlyweds jet to at last consummate their long-simmering love. Like many of his kind, Edward gets a little wild in the sack; he shreds pillows and destroys the headboard of a bed during their coupling, and bruises Bella all over her body. Happily for her, though somewhat upsettingly for the reader, she is so transported by erotic rapture that she fails to notice: ”I only remembered wanting him to hold me tighter, and being pleased when he did….” If this sounds steamy, rest assured (or don’t get your hopes up): Meyer writes about even furniture-wrecking sex with the decorum of a Victorian schoolmistress.

She is less restrained, alas, in her macabre descriptions of the pregnancy that immediately follows. Any reasonably astute reader will guess that Edward has knocked Bella up — inexplicable nausea, gnawing hunger — long before she figures it out herself. And it’s when Bella, suffering from morning sickness and gestating a vampire, starts vomiting ”a fountain” of blood, that Meyer jumps the shark.

The series has always been grounded in Bella’s human voice, which is imbued with adolescent fragility and unwavering passion for Edward, which, however goopily described, has a kind of winsome purity rare in young-adult fiction. You may wish she had loftier goals and a mind of her own, but these are fairy tales, and as a steadfast lover in the Disney princess mold, Bella has a certain saccharine appeal. As the masochistic teenage mother-to-be of a monster — a fetus that breaks her ribs when it kicks — she is not only hard to identify with but positively horrifying, especially while guzzling human blood to nourish the infant. (She adamantly refuses an abortion, which even Edward begs her to consider.) By the time the feverish birth scene rolls around, you’ll think Rosemary’s Baby might make a suitable companion video to What to Expect When You’re Expecting.

And this is just the beginning.

During the loonier stretches of the novel, Meyer wisely turns the narration over to Bella’s old friend Jacob, a warmhearted werewolf who has always been sweet on her. He becomes our tenuous anchor to sanity, as outrageous new plot twists sprout like kudzu. ”I felt like — like I don’t know what. Like this wasn’t real. Like I was in some Goth version of a bad sitcom,” Jacob confides before he too is swept up in the narrative mayhem. So do we, Jacob. So do we. D