You're free-falling through open sky with a few massive guns and a load of cute, gurgling babies being born by storks to their parents' waiting arms. It's your job to protect the bulging human race by slaughtering these kids.
Obviously you also have to contend with the massive, evil, fireball blowing, flying shark, that's a given. There may be some other evil to contend with too, but we don't want to ruin it.
Use the mouse to aim and fire, and '1-3' to switch weapons.