The choice that really is emotionally affecting comes before this. Do you kill the Big Daddy? You’re usually desperate for Adam, but these gentle giants are the only things in the game that mean you no harm. Once you’ve watched them hammer a few Splicers, you develop a real fondness for the big guys, and their death is far more disturbing than the mysterious vanishing act a harvested Little Sister pulls. Their tiny charge patters barefoot over to the enormous dead hulk, wailing and sobbing. “Mr Bubbles!” she cries, “Please get up! Please!”

BioShock’s main plot isn’t about the Little Sisters, but it does have a sequence that gave us a Schindler’s List pang of guilt for killing them all (we needed the Adam). And Schindler’s List isn’t a cultural touchstone that comes up a lot when talking about games. There’s a richness to BioShock’s fiction, a conflicted complexity to its characters, and a humanity in its themes that we’re wholly unaccustomed to in gaming.

But it is uniquely a game: its most powerful moments play directly on the conceits of gaming itself. Where others try to contort film scripts around interactive shooters, BioShock uses violence as a bloody foundation for its real stories. While the relentless onslaught of the murderously insane continually rams home the horrific nature of what Rapture has become, two other threads tell the story of its past and future.

The story of its past is something you have to investigate: it’s in the audio diaries left by Rapture’s citizens when they still had some marbles to lose. They tell their personal stories in instalments scattered throughout the game, so if you actively hunt them out, you end up following each person’s story to its (usually sticky) conclusion. Most of these are grim, some are achingly sad, and one or two are utterly nuts (look out for The Wild Rabbit). And one contains a harrowing twist to the main plot that’s revealed nowhere else.

As for what’s going to happen to Rapture, that story is the propulsive force of the game, and it comes to you over the wireless. Irrational don’t like to let you meet sane people in person - they can’t be simulated realistically - so don’t expect any Black Mesa East chapters. But there is one moment, in dealing with the few still-vaguely-cogent people of Rapture, that’s simply staggering to experience.

BioShock had already made us physically gape several times by this stage, but here our mouths fell open and stayed open, only widening further as the scene became more extraordinary with every passing second. We're going to suggest to Irrational that they patch the game to activate the player’s webcam at this moment, because the gormless visages it elicits must be hilarious.

Again, it’s BioShock smartly exploiting its status as a game for a psychological sting. That single scene casts the whole game in a new light, even your own actions within it. Irrational have somehow become such masters of game storytelling that they can toy with the very process, mock it, and bend it to their will.