It's a mysterious, moody thing that puts the viewer in the shoes of Johansson's workmanlike body snatcher, looking at Glasgow through the eyes of a creature that's never seen this particular civilization before. The fact that we can't understand what the Scots are saying through their thick brogue seems to be by design, and helps with the general feeling of dislocation. This is a movie about alienation that drives home its point with real aliens.
Early in the film, Johansson picks up an ant, studies it at the end of her finger. She looks at her (exclusively) male subjects in the same way - and it's that detachment that gives the movie its simmering horror. There's a truly hair-raising scene here of an indifferent Johansson happening upon a drowning incident at the beach, dragging off one of the victims, leaving a orphaned toddler wailing on the shore.
Mostly, we watch Johansson driving a van through the gray, cold, rainy streets, past bland modern structures, while in the background we hear the electrical/industrial/mechanical thrum of an urban place.
Just about the time you're thinking that the whole things is very David Lynch (don't expect the movie to make literal sense), up pops a man with a facial deformity, and the association is complete.
It's this "Elephant Man" interlude that helps the movie pivot. We understand that to this visiting alien, the man looks no better or worse than any other human. The alien senses how her attention makes the lonely man feel, and we see (Johansson's performance is quietly very good) her attitude toward her man-gathering project (and lonely planet in general) begin to change.
Do not, however, imagine that this portends a sentimental ending. "Under the Skin" does not repay her empathy and curiosity in kind.
Jonathan Glazer ("Sexy Beast") isn't that kind of director, we're not that kind of species.