of the Body Snatchers. The Stephanie Meyer connection was only brought up later.
Aliens invade Earth, but those aliens are some large blobs of goo that infect people and take over their minds.
One of them invades Saoirse Ronan, but she is special and both minds coexist. Her body flees to the human resistance, and now they must forge a coexistence between the species. They are thwarted by a alien-invaded Diane Kruger.
From that potential backstory and world building, what results is a piss poor love triangle and a bunch of convoluted ways for them to snuggle in the most overused stretch of desert just outside LA.
Andrew Niccol directed this, hopefully for the money and for the pleasure of having all the baddies driving stylish silvery Lotus Elises.