A man with a bizarre, degenerative illness travels with his wife to a remote island where, he believes, his ancestors lived. It's a desperate last attempt at figuring out what's wrong with him - he hopes that by tracking down his real family (he was adopted, you see) he'll be able to find out and thus cure the ailment. Unfortunately, he probably would have been better off not knowing, since it turns out his ancestors were a load of narcissistic inbreeders, and most of them are a lot worse off, genetically, than he is.
Written down like that, it doesn't sound too bad. But when you see the genetic 'monsters', and then are asked to believe that they're scary, rather than just pathetic, you'll spot the problem. The idea that the people living on the island had never noticed, or never done anything about it, is also problematic, as is the pseudo-science behind exactly what it is these creatures need to eat to survive. Clue: they ain't vampires, but you'll wish they were.
The ending is also complete nonsense; almost upsettingly so. Really, the whole film is ridiculous - ideas that look horribly disturbing on paper have become farce when converted onto the screen. The best thing about this movie is that it's given me a weird and slightly awesome expression to use when faced with unappetising food: "I'd rather eat my own pickled brother."