In Astrophage, you are an explorer who finds himself on the edge of a hostile planet determined to fight him off. Shoot your way to the center of the planet, moving through a maze of ventricles and arteries, to kill the heart at the center. WASD to move, mouse to aim and fire.
The rest of Booker's kind escaped the wheel of life, but he chose to stay behind. Now trapped on his home planet populated by demons, he is perpetually trying to leave the planet, but always crashes back down to earth.