From the box art, which displays a youngish sort of teenage girl with a wistful look in her eye, bordered by two random young hunks who look ready to fight to the death over her, to as many possible mentions of the Twilight Franchise and this movie being based on a work by Stephanie Meyer, anyone can tell immediately that this film is one of the most committee developed films of all time, designed soullessly to capitalize on a specific market and take as much money as possible before they grow older and realize exactly what has been done to them. You have to admire the skills of these Hollywood people in some ways.
Melanie (Ronan) lives in a world in which formless aliens have invaded, who then proceed to take over the bodies and minds of human hosts they inhabit. The word the movie uses is “soul” which sounds like a terribly blend of science and religion, but there you go. This is a slightly original concept, used notably before in the world famous young adult novel series Animorphs, released a decade and a half ago. But anyway. Melanie gets bonded with a soul named Wanderer, and given an assignment to infiltrate one of the last homelands of free humans. However, through shoddy character and plot design, Melanie of course keeps her own personality and shares her body with the alien. From there, she gets embroiled in (of course) a love triangle with two hunky men, complicated by the fact that the alien also has an attraction to one of these men. As you do.
The best thing I can say about the novel for the Host is that it is mildly better than the Twilight books, which I did indeed read to build an accurate opinion for myself. However, the movie steals whatever soul there might have been (ironically, might I add) and replaces it with pure pre-teen money printing magic. Everyone should avoid letting their young female children anywhere near it.