Where Is the Third King?

Polish Militia (police) learns about a planned theft on an old painting from a provincial museum in a lonely palace. A duo of police officers under cover of a married couple travels there to put a trap and replace the painting with a copy first. Surprisingly, there occurs a murder with a medieval weapon, and a suspect is among museum workers. In addition, there are more copies of the painting revealed. The question is, who is a killer, and where is the original.