This movie gives a whole new meaning to Two Girls One Cup. Yuck. According to Shock 'Til You Drop:
Two American girls are on a road trip through Europe. In Germany they end up with a broken car in the woods. They search for help and find an isolated villa. The next day they awake to find themselves trapped in a terrifying make shift basement hospital along with a Japanese man. A German man identifies himself as a retired surgeon specialized in separating Siamese twins. However his three "patients" are not about to be separated, but joined together in an horrific operation. He plans to be the first person to connect people via their gastric system (yay, ass-to-mouth fusion!), in doing so bringing to life his sick lifetime fantasy "the human centipede".
So in other words, the lunatic sutures their faces to the other person's rectum and urinary tract so when one goes, they all go in each other's... you get the picture. I'm assuming the head centipede will be on a clear liquid diet since the two pulling up the rear (so to speak) would be dead from dehydration in a matter of days. Yuck.