En route to Chicago, a bunch of young friends experience engine trouble and stupidly accept a ride from a passing trucker. He traps them in his trailer. Stuff happens. There's some vampire/monster/creature thingies that attack them. Some of the teenagers die. Yadda, yadda, yadda...
I almost rated Prowl 1/10, not because it's particularly awful, but because I've had a gut-full of mediocre modern horror as of late, and this was very nearly the final straw that broke the camel's back—just another forgettable, bland, derivative scary flick with a pretty female protagonist, an unimaginative threat, lots of dreary and often shaky cinematography, predictable loud noises to make you jump, and a 'surprise' ending that's not all that surprising. It's been less than a day since I finished the film, and I cannot recall a single image that impressed me enough to bother retaining it in my mind.
To be fair, Prowl is probably worth a 4 or 5 out of 10 (although I'll err on the less generous side): it starts out promisingly, is technically proficient, the cast do a good job of looking terrified even if I sported an expression of abject boredom throughout, and there's also some pretty effective gore. But in this day and age where horrors proliferate, a film needs to be so much more in order to make a lasting impression.