This anime deceives most people. The cute characters point to a love-oriented show for kids. Of course this is totally wrong. "Vampire Princess Miyu" is one of the most well-designed gothic series ever made. The plots are decent, but the real treasure lies in the themes and thoughts the show invokes.
Basically, the plot involves a vampire princess who was born a human. Miyu, the heroine, was given her vampiric powers through magical means in order to make her a guardian against demons. Part of her curse is her thirst for blood and her inability to age. Miyu is permanently young. As part of her quest, she protects and sends stray demons (called "Shinma") back to their dimension. She has several companions (good shinma) and a group of unknowing school girls as friends.
If you're only looking for action you might be satisfied. The fight scenes only occur at the end of each episode usually and are very similar to "Sailor Moon" though quicker and less showy. You don't see anyone transform. You do see gaudy introductions of monsters which are called by name. I suppose this is a widely accepted format in Japan and is used out of tradition.
For those who like complicated plots, look no more. You'll find all the twisted relationships, backgrounds, and flashbacks you'll ever want to see. The time span of the show is several hundred years and covers not only Japan but most of the world at some time or another. But most of the action and drama occurs in Japan.
I'd recommend not watching the last few episodes if you get into the show. The ending is terribly sad and does not follow the themes of the show very well. It's also terribly gaudy and poorly animated. Other than this the only thing I can relate about the ending is that it's just very strange.

- Necro-M