It could have gone so, so wrong, this Harry Potter thing. The best-selling book series in the world contains lines of dialogue like, “MURDER! MURDER IN THE BATHROOM!” and, “You are setting too much store by the prophecy!” (emphasis not added). Its main characters include a wise old wizard who enjoys alliterative aphorisms, pensive beard strokes, and—hold on to your copy of “Lord of the Rings”—sometimes carries a cane. The stories’ emotional climaxes tend to be accompanied by the sound of mermaids singing. And this is all deadly serious.

J.K. Rowling continues to live on the brink of death-by-overkill in “Harry Potter and the Half-Blood Prince,” her latest—and penultimate—addition to the series. Lord Voldemort, the evil wizard who has been trying to kill Harry since he was born, has gained a host of new advantages, including a new posse (a pack of “dementors”: creatures who suck souls out of the living, have abandoned their jobs as prison guards, and now roll on the dark side). And, as always, Voldemort has that most critical asset: wicked smart magic skills.

But not to worry, we are told, because Harry, our hero—now called the “Chosen One”—has something even better: the power of love.

“Just love?” Harry asks Dumbledore, the white-haired wizard.

“Yes,” Dumbledore replies. “Just love.”

It’s a miracle, really. And the greater miracle is, we believe it—and sometimes even cry when the mermaids come out. This is a pretty incredible feat, given how hard it is to be a self-important epic these days. We forgive Keanu Reeves when he babbles on about spoon-bending, fate, and being The One. Minus advanced digital kung-fu technology, however, the going is tough—at least in most blue states, and certainly among a generation reared on Mary-Kate-and-Ashley eye rolls and “SpongeBob SquarePants” absurdity. But even the hip 20-something in ironically oversized sunglasses I saw crossing D.C.’s New York Avenue the other day held a copy of “Harry Potter and the Half-Blood Prince” along with her Jana Feifer purse.

How has J.K. Rowling won us over? Partly by disguise. Rowling dresses her dead-serious battle of good versus evil in the sensibilities of the ironic age. “Anyone we know dead?” Harry’s best friend, Ron, asks every morning when the Daily Prophet flies to their Hogwarts breakfast table. Ron’s mother, Mrs. Weasley, has always had a magical nine-handed clock to monitor the whereabouts of every member of her family (“home,” “school,” “work,” “traveling”). In this book, all nine hands point to “mortal peril.”

Layering the seriousness of an inevitable life-and-death showdown with the day-to-day absurdity of flying newspapers and nine-handed clocks is like a spoonful of sugar to help the medicine go down. But it’s also like, well, life. Although Rowling’s plot has been, for the most part, mapped out since 1997—she forecasted danger before even Richard Clarke—she says she writes each book fresh as she goes, so it’s fair to call this what it is: post-9/11 fiction at its best.

In “Half-Blood Prince,” Rowling is also at her personal best. The author’s intricate plots have always carried the series, keeping us involved despite sometimes painfully plain prose. This time, the plot gets even better. Her technique works according to Chekhov’s formula—if there’s a gun in the first book, you can be sure there will be a death by the sixth—except J.K. Rowling doesn’t do guns; the clues she plants all have to do with biography, motive, and character. Earlier books had more fight scenes. In this one, Rowling pays lots of attention to questions like, “Why is Voldemort evil?” and none to questions like, “What sweet ninja skills does Voldemort have?”

It’s a welcome development, and one that suggests an answer to the nagging problem of why in the world we need another epic saga of good versus evil: we never had a female author before. Harry spends a good portion of this book not valiantly fighting Voldemort’s Death Eaters, but sitting still in an empty office, head immersed in a bowl of liquid, thinking. Of all the cool magical tools Rowling has imagined, this one—called the Pensieve—is probably the coolest. The Pensieve is like a flashback on drugs; instead of hands waving across a stage, “Wayne’s World”-style, journeys to the past are elicited by trips inside its pieces, conveniently liquidized for easy, do-it-yourself head-dunking.

Harry spends a good portion of this book thus immersed, as Rowling steers the story into a piecemeal biography of Lord Voldemort, building vignettes from the dark lord’s youth as a wizard named Tom Riddle.

Uncomfortable questions follow. “Know thy enemy” is Dumbledore’s ostensible rationale for escorting Harry through his nemesis’ past. “How else will you find out his weaknesses?” reasons Hermione. But then things start to get tricky, and it becomes clear Harry isn’t just learning where to hit Voldemort when he’s down. “Could you possibly be feeling sorry for Lord Voldemort?” Dumbledore asks, catching him solemnly wondering why Tom’s mother left him parentless. Harry does not answer, but the question lingers: Is it possible the dark lord deserves our sympathy?

Blending the coming-of-age trope with the dark-versus-evil one can introduce stunning problems like that. Unfortunately, it also makes it doubly easy for Rowling to lapse into cheap genre gimmicks.

In “Half-Blood Prince,” Harry “Power of Love” Potter and friends are suddenly and embarrassingly horny. Sometimes this is (literally) charming: when her love interest hooks up with another girl, for instance, Hermione conjures a flock of twittering birds to attack him, and they batter him all the way to the door “like a hail of fat golden bullets.” But elsewhere the romance feels uninspired; Rowling is apparently a little too far away from adolescence to remember youthful flings as anything more than triumphs in physical awkwardness, as scene after scene of 16-year-olds “eating” each other’s faces makes clear. She may have also finally exhausted the magic jokes. It is time to consider abandoning your premise when “A Cauldron Full of Hot, Strong Love” is the wittiest song title you can come up with.

But we forgive those occasional lapses (and the occasional overwrought speech of Dumbledore) because Rowling has more important things to focus on. It probably does all does boil down to love, like Dumbledore said, but that turns out to be a more complicated proposition than you might think. For Rowling, love is not just a “force” to be accessed in order to turn on your light saber, something some of us have been blessed with and others born without; it can also be a choice, an impediment, and even—we are led to believe—a weakness. Exactly how much a weakness and how plausible a choice is something Rowling will have to resolve in the next book, and, for that reason, the final installment promises to be her most ambitious yet.

No matter how many times the word “murder” appears in all capital letters, this story will only get better. Let’s just cut down on the snogging next time.

—Staff writer Elizabeth W. Green can be reached at egreen@fas.harvard.edu.