Imagine a world without war, its inhabitants living together in perfect harmony.

That dream is realized in Stephenie Meyer's The Host, a silly, if fun, futuristic sci-fi romance that follows the megaselling author's Twilight film series.

Trouble is, the Elysian future depicted in The Host was created not by us, but by aliens who have taken over the planet and enslaved its human population.

The first in a planned film trilogy, The Host was adapted from Meyer's 2008 novel by sci-fi specialist Andrew Niccol (Gattaca, In Time) and coproduced by Meyer herself.

It's about a race of aliens named Souls (yes, it's a Deeply Symbolic name), these small silvery beings who look like waterbugs made of tiny fiber-optic cables, who can survive only by colonizing other creatures. They crawl into the back of our necks and take us over, Invasion of the Body Snatchers-style.

Once inside a host, the Soul takes over, killing off the original personality.

They're not such bad fellows: Peaceful to a fault, considerate, always mindful of the common good, they transform the world into a New Age colony populated by Valium-ingesting Stepford Wives. They wear immaculate white outfits and drive cool cars. By comparison, the human survivors live savage, angry, sweaty, smelly lives.

The story opens with a chase: A group of Seekers (mind the symbolic word) - they're the Souls' police squad - are chasing a group of survivors.

They capture the feisty Melanie Stryder (yet another symbolic name) and force her to become host to an alien waterbug chick named Wanderer (hmm, interesting name).

Played by the remarkable 18-year-old Irish actor Saoirse (it rhymes with inertia) Ronan, Melanie is stubborn, and her identity survives.

A battle ensues between the two personas inside her, until they begin forging a kind of understanding, a sort of sisterly love.

Wanderer helps Melanie escape to join a colony of humans who live in a system of caves in the Arizona desert. William Hurt is suitably kooky, avuncular, cranky, and bossy as the colony's leader, and Melanie's uncle, a hippie scientist-type named Jeb Stryder.

Deeply riven, confused, and frightened, Melanie has a hard time reconnecting with her love, Jared (Oscar winner Jeremy Irons' son, Max Irons). Her inner alien makes things more complicated when she falls in love with Jared's best friend, Ian (Jake Abel).

The Host has its high points, but it is for the most part deeply goofy.

Meyer's story does pose interesting questions about the nature of human identity (are we our minds or our bodies?) and that perennial chestnut, Can humans cohabit without hatred and violence?

To Niccol and Meyer's credit, The Host steers clear of Michael Bay-ish explosions, instead focusing on its characters. But the pace is too lackadaisical, the characters too laid-back. It's hard to believe that Jeb's group is supposed to live in constant fear and mortal dread.

Worse, The Host isn't very good as a romance. There's not very much passion on screen, which makes the film a poor cousin to Twilight.