In Amsterdam, according to legend, St. Nicholas comes around on December 5th to give out presents to all the good boys and girls. He is assisted by Black Pete, who drops the toys down the chimneys. It is said that the bad children are taken by St. Nicholas and taken back to Spain on his boat.
The true story is that Nicholas was an evil former bishop who was murdered along with his men on December 5, 1492. The townsfolk were tired of giving him all of their money, valuables, and virgins, so they burned Nick and his men alive on their boat. Now, "St. Nicholas" appears whenever there is a full moon on December 5th, along with his "Black Pete" sailors, and kills as many people as he can while stealing their children in revenge.
The last time this happened, a young boy named Kurt witnessed his entire family murdered by Nicholas and his men. Now, Kurt is a police officer when there is another full moon on December 5th. St. Nicholas and the Black Pete's come to Amsterdam and start killing people, including the friends of a young student named Frank.
Frank and Kurt team up to destroy St. Nicholas once and for all. Kurt has rigged up a boat full of explosives, which he plans to use to blow up Nicholas's boat with all his men on it. Kurt is mortally wounded by Nicholas, and Frank has to blow up the boat. The boat explodes (apparently, with hundreds of missing children on it). St. Nicholas disappears into the night.
The next day, the news is full of cover-up stories about all the missing children (according to the mayor, this is a regular occurrence, and has been for centuries). Frank is recovering in a hospital, and gets a large check to keep quiet. However, St. Nicholas is still alive, and can be seen at the end of the movie, perched on the top of a massive cathedral.
Thanks Paul B!