1960. The Pacific. Your plane has just crashed into the ocean, but incredibly, even though you’re thousands of kilometres from land, there’s a lighthouse nearby. You pry open the hatch and descend into a vast underwater metropolis of leaking, rusted laboratories and moldy Art Deco apartments. Flickering lights reveal decaying corpses everywhere, many looking not quite human.

You’ve found Rapture.

After WWII, Andrew Ryan built this undersea colony as a capitalist paradise for select scientists and industrialists; a place where society’s elite could enrich themselves without the burden of caring for their inferiors. Then things went terribly wrong in Ayn Rand’s — er, make that Andrew Ryan’s — utopia.

Rapture’s researchers, you slowly learn, discovered “Adam,” a substance allowing instant, controllable genetic mutations. Soon civil war broke out over the stuff, with factions mutating their forces in increasingly powerful and disagreeable ways.

Today Adam is harvested from the dead by — take a deep breath — pigtailed eight-year-old waifs with glowing eyes, wielding veterinary-sized syringes. These are the Little Sisters. They giggle and scamper. They are deeply, viscerally unsettling. Worse, Little Sisters are shadowed by Big Daddys — nearly undefeatable bodyguards, faceless under diving helmets.

Good thing you’re handy with a gun. Despite survival-horror and adventure elements, BioShock is a first-person shooter, but one so ambitious some are calling it an “FPS 2.0.” In recent games like Gears of War, computer-controlled enemies have become very good at searching, attacking and defending, but here they can assess threats and make targeting and tactical choices. With their symbiotic devotion to their Little Sisters, this makes Big Daddys more vulnerable to guile than grenades.

While shooters follow a very linear path, this story is fluid, like a role-playing game. Everything that happens results from your actions, and encounters rarely play out according to a set script. There are still creepy corridors, gory visuals and an array of imaginative weaponry, but instead of wasting every mutant you see, BioShock asks you to consider the morality of taking one life to save your own. Can you really kill an enemy with the face of a little girl just for a power boost?

When hard-boiled P.I. Fred Neuman literally loses his head in a murderous mishap, he’s resurrected by a mad scientist, but without his memory…or his cranium. Understandably irritable, Fred begins a quest for revenge, gaining supernatural power by wearing the severed noggins of defeated foes as his own.

The third-person action is set in an expansive world straight out of a 1940s film, but with bizarro touches, like ghoulish gangsters using mutants and zombies to control the grimy, nameless city. And while solving his own murder, Fred — voiced by John C. McGinley, the cranky Dr. Cox on TV’s Scrubs — will become quite the expert decapitator himself, using his various heads for both nifty combat moves and mini-games like “Dirty Pool,” “Head 2 Head Pinball” and “Consensual Sax.”