At Mordred's secret lab, when the owl flies out, you feed the mouse to the mechanical owl. The owl becomes your friend and follows you.
Click on his card in your items bag.
release it to the owl. then the owl will follow you
It is a mechanical mouse. (see related question)
The mouse will get surprised, run around, then run back to you.
You scare the owl outside. follow it and release the mouse. it will know follow you
Click on the "bag" icon (your items) and chhose the card with the mouse. If you release it near the owl, it will be eaten (chomp). Thereafter the owl will follow you in the underground passages (but not in the castle).
Feed it the mechanical mouse (from the Maid's room) and the owl will follow you.
After you get the cheese you put it in the princesses dressing room and capture the mouse to feed it to the owl.
To catch the mechanical mouse, in the Princess's Maid's room (castle first floor left).
the mouse is in the princesses room in the hole in her wall but you have to get the cheese to get him/her.
In the Maid's room (castle first floor left) you will find a mechanical mouse.
when an owl pops out near rumor mill