You and your scientist father are almost getting hanged for "witchcraft", but you manage to escape with the help of ingenious magic (smoke bombs).
You both hide in the woods (maybe in an abandoned cabin) and start working on a plan to get out of this misery.
Your solution: Change the king's mind. How? With The Beam o' Mindchangin' (of course).
After a few hard days of work, your father flips a switch on a giant machine and the blinding beam gets shot at the door.
This is where you begin playing. Your goal is to get the beam to hit the king's head. Unfortunately, the device is way too heavy and can't be moved (think of the first computers ever created).
Your father gives you a helmet so that you're safe from being hit with the beam. You open the door and the beam bursts outside. You follow.
The game is - just like the 2D Zelda games - played in "rooms".
You have to redirect the beam using mirrors. Your father gave you ten mirrors to start with and get you through the forest to the village.
The village is where the fun starts.
You have to start looking for mirrors in every house, maybe even buy some using money that you have to steal.
You can use the beam on everybody else too, although regular villagers will simply run away in fear.
Guards will charge at you and try to arrest you (which will probably lead to your immediate execution) - and they shatter mirrors from a distance if they see one.
You have to lure them into a corner and maybe take away a mirror so that it hits the guards. Those guards are on your side from now on and they will protect mirrors for you.
It takes a while through the village, but the castle is even more ridiculous, it's similar to a maze, and at one section you may even have to redirect the beam outside from window to window.
The king is equal to a boss. You can find crystals and can spread the beam to have a higher chance of hitting the king with it.
In general, the map displays an overview of the whole area (village, castle, forest) and where the beam is shot through, which would surely look quite interesting after a while.
That is the whole idea. I think it wouldn't be that hard to implement. The artificial intelligence of guards would require some thought, but other than that, I don't see much of an issue.