Meet Wanda — short for Wanderer — and Melanie, both played by Saoirse Ronan. Wanda is an alien, essentially, and Melanie is human.(Photo by Alan Markfield)

Judging solely by the film adaptation of Stephenie Meyer’s 2008 novel “The Host,” the “Twilight” author’s commitment to the love triangle is inviolable. This sci-fi love story has way more Harlequin Romance in its DNA than Harlan Ellison. It’s a mushy and unsuspenseful melodrama.

Like the Edward-Bella-Jacob triangle that anchored the “Twilight” books and movies, “The Host” centers on a teenage girl (Saoirse Ronan) who’s torn between two lovers. Like Bella, she’s of two minds. Only this time, it’s literal.

One of them is called Melanie. Melanie loves Jared (Max Irons). But Melanie’s body has been taken over by a parasitic alien that resembles a fiber-optic caterpillar. The alien — one of a race of creatures known as Souls, who have taken over most of humanity — has its own personality and name: Wanderer, later shortened to Wanda. Wanda loves Ian (Jake Abel).

Wanda and Melanie spend most of the movie bickering with each other over boys, which makes Ronan look like a crazy person. What’s a girl to do?

That’s the dilemma at the heart of “The Host.” It tries, with less success, to gin up a little excitement around the fact that Melanie/Wanda has escaped from her alien overlords, but the stakes are too low to generate much heat. Despite their habit of crawling into people’s spinal columns, the Souls are a generally peaceful sort, preferring the kumbaya of colonization over killing.

This might make for a slightly creepy nightmare of possession, but it makes for a somewhat dull action flick, where the few deaths are more likely to be suicides by humans who’d rather drive a truck into a concrete barricade than share head space with a talking tequila worm.

One of the Souls is less friendly than the others. Played by Diane Kruger — in a performance that manages to be both robotic and scenery chewing — the Seeker is determined to use Wanda to track down and eliminate the human resistance cell where Melanie/Wanda is hiding with her two boyfriends.

Rather than offering any “Invasion of the Body Snatchers”-style thrills, however, “The Host” wastes most of its two hours on high-school-caliber drama. “You’re angry when I kiss a man you do love,” Wanda whines to Melanie, after smooching with both Jared and Ian, “and you’re angry when I kiss a man you don’t. It’s very confusing.”

It’s also fairly boring, despite the inherent novelty of watching an actress talk to herself, like Sybil. The gimmick probably worked better in print, where the different speakers could be conveyed with different typefaces. On the screen, it’s just goofy.

As for the acting, Ronan does a decent job of conveying her character’s split personality, at least compared to “Twilight’s” Kristen Stewart, who seems to have one speed: mope. And William Hurt is okay as Jeb, the leader of the human resistance. But much of the rest of the cast is wooden, and I’m not just talking about the pod people.

Writer-director Andrew Niccol (“In Time,” “Gattaca”) is usually better than this. But I don’t blame him. His body has been taken over by Meyer, whose monomaniacal focus on the supernatural menage a trois is guided by her own alien overlords: the teenage girls who buy her books by the millions.

PG-13. At area theaters. Contains sensuality and some violence. 121 minutes.