In the distant past, there was an age when demons ruled the world. A familiar enough notion. A time of terror, of "blood, flesh, and iron". A dark age for mankind. And amidst the horror, God in His mercy, or perhaps caprice, sealed all of the demons away in a certain box, never to trouble humanity again, save for a single woman, who out of the depth of her sin opened that box once more. Thereafter, that box came to bear her name.
The Violinist of Hameln (yes, that's the proper spelling -- look up the homepage of this real city in Germany if you don't believe me) is the saga of a world in which Pandora's Box is quite real, and where the forces of good are locked in a desperate struggle with the reawakened demons to prevent the resurrection of the Archdemon Chestra and the end of all mankind. The Violinist of Hameln began serialization in manga form in 1991 as one of the debut titles in Comics GanGan, published by Enix (remember the Dragonquest game?). At 22 volumes of tankoubon and still running, this work by Watanabe Michiaki is a shining example of how comedy and dire seriousness can be mixed to amazing effect.
A full summary of the plot is impossible in a short article, but a bit of background will help understand what's going on in the movie. At the beginning of the story, Haamel and Flute are two normal citizens of a small village in the countryside of the kingdom of Sforzando. Their peace is shattered when demon attacks, not seen in fifteen years, befall the village and propel the two on a journey to the capital city. Accompanying them is Oboe, an intelligent talking crow who has been with Haamel since his early childhood. Along the way they meet Raiel, another musical sorcery user who carries a magical golden piano with him. Upon arrival at the capital, it is revealed that Flute is actually the daughter of Queen Horn, and that Haamel has a dark secret in his past that links him to the impending struggle with the demons. It is also revealed that one of the four generals of the demon armies is a beautiful fallen angel named Sizer who has reasons of her own for fighting Haamel. After 11 volumes of manga and a lot of unpleasantness, Sizer switches allegiances and joins Haamel's party, hoping to atone for the sins she performed in her past. Thereafter, Haamel's party begins heading for the distant Northern Capital, wherein lies the seat of the demons' power and the key to the battle and Haamel's past. Waiting for them there is Bass, general of the Dark Magic army of undead and overall leader of the demons until the return of Chestra. It is at this point in the story that the Violinist of Hameln movie is set. Haamel, Flute, Sizer, and Raiel are four of the Five Great Hopes, the fifth of which (Tron Bone) does not appear in the film.
The Violinist of Hameln movie is quite short (roughly 30 minutes) because it was shown as part of a triple feature with two other titles serialized in GanGan: Dragonquest and Mahoujin Guru-Guru. The movie is relatively light-hearted since it's set in a comparatively peaceable part of the manga and was supervised by Watanabe when his writing for the manga was still in a "good" mood. After the release of the movie, a 25-episode TV series of VoH was made, under the direction of Imagawa Yasuhiro (who also directed Giant Robo the Animation). The TV series is much darker than the movie, and that same darkness is mirrored in latter parts of the manga, where terrible things start befalling the characters. I highly recommend all of the VoH canon, especially to people who are seeking an escape from the run-of-the-mill, happily-ever-after anime that pervade the industry today.
One other note: You may have noticed that a lot of the names in the VoH world resemble musical terms. This is 100% intentional. From the names mentioned above, it might help to note that (Synthe)Sizer is an instrument, and that the Archdemon ("Dai-Maou") Chestra makes more sense as a musical term in Japanese.