Stephenie Meyer's "The Host" promises much — alien invasion, near-subjugation of the human race, a plucky band of survivors holed up in the desert waging low-level guerrilla war on the invaders — and through 619 pages strives sincerely with the key ideas driving the book, which finally come down to "love conquers all."

Meyer is the best-selling author of three vampire novels, all for teen readers. Much is being made of this being her first "adult" novel. The question is, which adults? There is nothing in this novel that a sharp 12-year-old couldn't handle, especially one who has been reading science fiction.

As to that, "The Host" is, not unexpectedly, derivative. But compared with such estimable works as Jack Finney's "Invasion of the Body Snatchers" or Robert A. Heinlein's "Puppet Masters," "The Host" feels light.

The book opens with the surgical insertion of a Soul — a silvery parasitic life form — into the unconscious body of a human survivor, partly so the Seeker that caught her can find out from the human's latent memories where the rest of the humans are.

When the Soul awakens in the body of Melanie, she discovers that Melanie has not vacated the brain, as is usually the case. As the days pass, in fact, it becomes clear that Melanie has no intention of leaving, though she cannot regain control of her body.

Between Melanie's constant backchatter and the Seeker who suspects what is wrong, the Soul, now called Wanderer, flees.

Wanderer, with Melanie's sometimes grudging help, manages almost by accident to find the enclave of humans and becomes the center of a struggle among the survivors to figure out the right thing to do. She becomes enmeshed in a similar, though more personal struggle with Melanie because both Melanie's brother and the man she loves are residents in the desert hideout. Wanderer, who has taken Melanie's memories and essence for her own, finds that she is also in love with him.

A love triangle — two females in love with the same man — with an interesting difference, as the women have only one body for themselves.

The possibilities for a fascinating twist on Cartesian duality are richly present, but like so much else in "The Host" they are mostly glossed over. For all that the two minds share, they share no personal secrets, at least not the sort to make for penetrating psychological drama.

Compared with its many predecessors, the invasion described is benign. The justifications are brief and politic (the humans are violent and destroying the planet) and the actual takeover hardly examined. Given the nature of the invasion, one wonders how it would go down in countries other than the United States — for example, in Myanmar or North Korea. Such scenarios work so well here because of the profoundly porous nature of our society.

Meyer is careful and makes no false steps, but such caution only leaves one wondering at the novel it could have been had she written a genuinely adult book.

The characters are engaging and the questions raised (and unanswered) are good primers for denser examinations. As such, "The Host" would be a good first "adult" novel to give to that sharp 12-year-old — in preparation for the meatier works of those who have treated this theme before and those who will in the future.

Mark Tiedemann is a St. Louis writer, author of "Remains" and the "Secantis Sequence." He is president of the Missouri Center for the Book.

www.marktiedemann.com