I have mixed emotions about Isle of the Dead. On one hand, I have tremendous respect for producer Val Lewton and what he accomplished in filmmaking while working with a tight budget. The cinematography and lighting are done with great care and skill. [All his films look gorgeous.] Boris Karloff and the rest of the cast do a fine job in the acting department. The movie has an overall eerie mood as well.
On the other hand, there is nothing about this film that really grabs you. The pacing is rather slow and nothing much happens. Karloff does the best he can with the material at hand, but it feels like there's not much to work with here. This film is lacking some much needed tension and suspense. Where is Hitchcock when you need him?
The basic plot takes place in Greece during the 1912 war. A group of people are trapped by quarantine because of the plague and are unable to leave the island. As death begins to make house calls, a superstitious peasant woman suspects one young girl of being a demon called a "vorvolaka." The plot thickens, or doesn't, from there.
If you are unfamiliar with Lewton's work in the horror genre, Cat People (1942) is a better introduction. That being said, you can do a lot worse than Isle of the Dead. If all films were this well produced, we would be fortunate indeed! [You can rent this one through Netflix. The restored version from the Val Lewton Collection is a must.]
For more info check out the film's entry in IMDB.