Alright...I would not recommend anyone actually viewing this trailer or movie unless you are a seriously sadistic fuck. I did because I had to bite the bullet for everyone else; you are welcome.
Seriously. Don't watch it. It takes the movie genre of 'Torture Porn' way past the point of gross and sick, but still somewhat enjoyable and goes straight to the worst sadistic shit you could imagine. It's gross.
Basically two American girls on vacation in Germany get a flat tire (they are REALLY bad actors too) and, of course, they are "horribly lost" and it's raining and they come across a house where a crazy German doctor lets them in. He then proceeds to make them into a human centipede. You don't need to know more, just assume it includes gross things that will make the audience wretch.