Like all good tales, a protagonist must be accompanied by a antagonists. The main protagonist of the story was the third son. He was about to accept the little mouse as who she was before she turned into a princess. You can tell he is a good person because he was quickly became depressed that the little bridge was pushed over the bridge.
The problem of the story was that the third son or protagonist could not find a suitable human wife. He then decided to wed a mouse who could make fine bread and weave good cloth. At the end of the story, it is discovered that the third son did not marry the mouse. He instead married a princess who was the mouse.