Låt den rätte komma in (Let the Right One In)
Oskar is a twelve-year-old boy who gets bullied at school. What looks like a father and his daughter move in next door, and he makes friends with the girl, Eli. Eli is twelve as well, but she's been twelve a long time. Eli is quick to tell Oskar that they can't be friends, but they develop a friendship soon enough, and eventually Eli agrees to "go steady" with Oskar.
Eli as a vampire is not vicious or demonic most of the time. She's just a girl who has a sickness and needs a particular kind of medicine, as it were. When she feeds, however, something changes. She becomes like an animal. Oskar is afraid and a little repulsed when he finds out what she is and what she has to do to survive. And Eli is left alone when her "father" (not actually her father, but more on that in a minute) offers her his own blood because he can no longer kill for her (as he's been doing throughout the film). Oskar has a deep affection for Eli despite his misgivings about her abnormality, most memorably after he taunts her about having to be invited into his home and she shows him what will happen if she comes in without the invitation.
And Eli is a rather kind, compassionate friend for Oskar, which is something he needs. She encourages him to stand up to the bullies at his school, and he eventually does, hitting a particularly nasty little boy so hard he becomes deaf in one ear. This creates the conflict that comes to a head in the film's climax, where the kid's older brother seeks retaliation.
The vampire lore is treated rather loosely in this film and tweaked for the demands of the story in most cases. We're not told how Eli became the way she is - presumably she was bitten, as we see another character take on vampiric traits after being bitten by Eli. She's not dead, as many vampire tales establish the host body to be. But she is rather strong, and has an animal-like ability to climb.
On one level, this is a sweet love story, but there's another, much more melancholy layer to it that's not explicitly stated, but is obvious once you follow the logic through. See, Oskar is going to age, while Eli will not. Their relationship will cease to be a romance once the age gap widens, and Oskar will be just like the man Eli lived with in the beginning - appearing to strangers to be her father and forced to kill so that she can have blood. The ending seems happy and sweet, with Oskar on a train with Eli hidden in his trunk and the two of them passing Morse code messages to each other. But once you think about what the future must hold for them, the story takes on another tone altogether.
This is SUCH a wonderful movie. Perhaps the greatest film ever to be made about vampires, and I don't use that superlative lightly. It's being remade in English, and I'm glad they're at least casting kids the same age as the Swedish version instead of smexy 20-somethings. But I can't imagine a better version than this one.
For your viewing pleasure, the famous swimming pool scene, wherein we learn what happens when you mess with Eli's best friend. (Somewhat graphic, but mostly suggested, off-screen violence.)