You’re a simple character living in the simple x-y plane of Flatland. You can move anywhere on the x-y plane but have no inkling of any other dimensions. One day, some 3D fucker lifts you out of your plane and drops you onto a different one. You must traverse back through the sheaf of 2D planes to return to your home plane. Better hurry, though. There’s something terribly… wrong… with these ones.
Your character keeps going forward until the button is pressed. As long as you are holding down that button, you will stop moving forward and spin in place. Upon releasing the button, you keep going forward in the direction that you are facing.
You emit a light and can see everything immediately around you–everything else is dark. The camera is centered above you looking down and follows you on your journey.
There are portals? gateways? friendly 3D’ers? that can help you travel to the next dimension down. They also emit a light.
There are terrors in the darkness–they do not take kindly to strangers in their midst and will destroy you if you let them get too close.