You have to pick up the mice and bring them safely to the left side of the building where the steel fire escape stairs are located. When all the mice have left the building trough the emergency exit at the bottom-right of the screen it is your turn to leave the building by using the stairs that appear randomly on the lowest platform level. You progress to the next level when you have left the building.
The shape of the platform changes each level and also fire balls will appear making it more difficult to you to rescue the mice. When a fire burns too long on a particular part of floor that floor will disappear and don't get caught by the fire or you'll lose a life.