Nestled in the mountains, the door that leads to Vault 11 is the kind you see hammered onto a shack. It is rotting wood and nails and spider webs clinging on to existence like the people of Fallout's post-apocalyptic Mojave Wasteland. The Brotherhood of Steel have sent us here to find a differential pressure controller, one of the parts needed to repair their faulty air filtration system.

The Brotherhood said nothing about the horrors inside.

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Vault 11 is unusual in that when you first arrive its main door, the one with the number 11 on it, the one would normally hiss and creak before pulling back and rolling sideways, is open. Most of the vaults you encounter throughout the Fallout games are locked shut, which makes sense. The vaults were designed to keep nuclear fallout out and happy dwellers in. Why is Vault 11's door open now?

Through the main door, on the floor of a large entry room are four skeletons huddled together. Next to one is a 10mm pistol. Nearby is a terminal, one of Fallout's famous green-flickering lore boxes. On it is a security recording of the vault entrance.

Voice 1: Are we really gonna do this? It's open. We could just leave.

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Voice 2: I couldn't. Not after that.

Voice 3: We don't deserve to leave.

Voice 4: "A shining example." That's what it called us.

Voice 1: But we were! We did what we were supposed to.

Voice 5: Not by a long shot.

Voice 1: Anybody would've done what we did.

Voice 2: You ask me, that's exactly the problem. Now let's get on with this.

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Voice 3: I'll go first.

Voice 1: Wait. Wait. People should know what happened. They could learn from it.

Voice 5: If there's anyone out there at all, I hope they never have to find out. Ready, Harry?

Voice 3: Yeah.

Voice 1: No, no, no, wait!

Four gunshots in succession.

Voice 1: Whimpers.

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Sound of a pistol dropping to the floor.

End of recording. What happened here? What drove four people to want to kill themselves? There are five voices on the recording but only four skeletons on the floor. What happened to the survivor of this apparent mass suicide? What happened to Voice 1, the whimperer?

Splattered on the walls of the vault are posters - propaganda for what sounds like an election. But this election is different. It encourages you to vote for candidates who are, it seems, horrible, horrible people.

"Don't vote Glover. His family needs him!"

"Haley is a known adulterer and communist sympathiser. Elect Haley for Overseer."

"Rumours about Haley are baseless. Vote Stone for Overseer!"

"Glover has done nothing wrong. Vote for Stone!"

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"I hate Nate."

Vault 11 initially baffles, but what's clear is its inhabitants were in the run up to an election to find the next overseer. This election, however, looks like it was broken. Instead of the election being about finding the best person for the job, it seems to have been about finding the worst person for the job. Why, exactly?

Vault 11 is my favourite vault in the Fallout series. When I first encountered it, in the thrilling days after Obsidian's wonderful Fallout: New Vegas came out in 2010, it took me by surprise. Its story unfolds like a horrific picture book, each room and corridor and terminal adding yet another layer of dread. And it made me think. It made me think about what it means to be human, the good, the bad and the very, very ugly. Eight years later, Vault 11 remains firmly rooted in my memory, as if it were a real place I once stumbled upon during a drug-fuelled road-trip from Los Angeles to Las Vegas, the kind of trip you taste on your tongue when you remember it, so stark were the events.

Like most vaults in Bethesda's Fallout games, Vault 11 is a maze of decaying corridors and stairways. Everything from the toilets to the clinic looks worse for wear, as if the walls and ceilings were dripping with entropy. Maintenance, a classroom, living quarters and all the other rooms needed to keep a small community of vault dwellers alive and kicking are present and correct - and creaking at the seams. Much of the place is window dressing - virtual innards that exist because they have to, because a vault needs to have these places for it to feel real, lived in and in the case of Vault 11, seemingly abandoned.

As always, the terminals hold the key. On them you find the Vault 11 election guide - a "handy" Dweller's Official Guide to Obtaining Overseers Democratically, or D.O. G.O.O.D. In it each of the three candidates is outlined, including a personal message. Here's Henry Glover, for example:

"I'm a devoted husband and father of six beautiful children. My oldest, Sam, was on the honour roll this quarter, and I couldn't be prouder of him. My youngest, Henry Jr., just said his first word, and it was 'Da-da.' We've got this bond already and he's still just a baby. Friends, when you go to the polls this election, I want you to think of your own children. Then I want you to think of Sam and Henry Jr. Picture their faces. Nate Stone should be overseer, not me."

The other candidates sound similarly desperate not to be elected. Why? You'd imagine the job of overseer would be one everyone wants, after all, the overseer is the boss, and you even get your own office. In Vault 11, though, things appear to work differently.

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We then discover the upcoming election was postponed following a spate of murders. "God willing, if the killer is apprehended swiftly, we may have found a promising new candidate for overseer," writes Terry Hart, president of the Human Dignity Bloc.

Why would you want a murderer as overseer?

And then we read Overseer Order 745, penned by then overseer Katherine Stone, the wife of Nate Stone. As overseer, Kate scrapped the traditional democratic method of electing a new overseer each year in favour of random selection, "ensuring complete impartiality and fairness."

Downstairs, on the second floor of Vault 11, we find the personal terminal of Roy Gottlieb, the head of the Justice Bloc and the Chairman of the Coalition of Vault 11 Voting Blocs. On it is another recording, where two voices, one of which belongs to Roy, discuss Katherine Stone's termination of the election process. Roy wants to revolt, so unhappy is he with the random number generation Kate has put in place. This revolution would involve arming up to force Kate to return to the previous system, Roy reveals, but his companion expresses concern the other blocs won't tag along. The Justice Bloc, it turns out, has the majority and thus the political power in Vault 11 which has, for some time at least, meant they've been able to influence the annual overseer election. But if Justice Bloc take up arms, will the other blocs play ball or fight back?

It's here the story of Vault 11 takes a disturbing turn.

In security is a terminal that is home to a file called deposition. This is an excerpt of the deposition of "defendant" Katherine Stone by Vault Attorney Gerard Miles. In this conversation, Kate suggests the Justice Bloc were planning to endorse her husband, Nate as a candidate for overseer because he had beaten some of its members at poker. Kate then accuses Roy Gottlieb of asking her for "sexual favours" in order to prevent Nate from being endorsed. Sexual favours not just for Roy, but the entire Justice Bloc leadership and their friends.

Kate agreed. "What else could I do?" she says. This gang rape goes on for a month before the endorsements are released. Kate's husband, Nate, is on the list - despite the horror she had put herself through to save him.

Kate admits to being behind the murders that forced the election to be postponed. Enraged by the Justice Bloc's actions, she killed some of their members to thin out the leadership and reduce their majority. She reveals she expected to be caught. That was her best chance. Now the people will elect her, she says.

"A confessed murderer?" Gerard Miles utters in astonishment. "You think voters would be willing to risk putting you in charge?"

"They have to pick somebody and live with their reasons," Kate replies. "Wait and see."

Kate was right. She was elected as overseer, and immediately changed the rules.

At this stage, we still don't know why the inhabitants of Vault 11 are terrified of becoming overseer, but we do know that a new overseer is elected each year. Why? What happens to the old overseer?

Despite Fallout New Vegas being eight years old, the divisions that strike through Vault 11 feel entirely relevant in the here and now. In the age of Brexit and Trump and a politics most feel has spiralled out of control, Vault 11 and its bastard blocs and dirty elections all of a sudden feel uncomfortably close to home. I'm not sure what the moral of the story here is. I'm not sure The Wasteland is big on morals. I'm not sure the real world is either.

Deeper within the vault we find the reactor room, which is packed with skeletons hidden behind sandbag barricades. A battle took place here, but between whom? Was this Roy's rebellion? It looks like it. It looks like the bloody result of Roy's personal terror at the prospect of becoming overseer because a computer might randomly select him after years spent in control of his own fate.

In the atrium we find a terminal home to the prepared speech of Gus Olson, ombudsman for the annual overseer election. Olson ponders the election process, and what it really means for the people of Vault 11.

"We want it to make sense. To understand why the vault's mainframe will kill us if we do not offer one of our own as a yearly sacrifice. To fully comprehend why we continue to have these elections despite the unfettered corruption that has plagued it for what must be decades by now."

The vote for an overseer is the vote for a sacrifice. Vault 11 is an election for an execution.

"I can only wonder if there are no answers to be found," Olson laments, "and we are just going along with this because we don't see another choice."

Vault 11 is expert video game storytelling on the part of the designers at Obsidian Entertainment. Events are revealed slowly. It demands a patience on the part of the player. Much time here is spent exploring dead space, areas of the vault that contain no important documents, eye-opening terminals or NPCs who conveniently spell out the past for the bored or short of time. Instead, we pick up fragments of story via dweller logs and recordings, wonderfully written and then thoughtfully placed so that the player is likely to encounter each in the order planned by the designers. Environmental storytelling is used to good effect: the propaganda posters, the placement of skeletons, a solitary handgun, an abandoned room where the corrupt politics was conducted. We absorb Vault 11's truth via virtual osmosis. Then Vault 11 comes full circle, its conclusion leading into its introduction. It is a satisfying payoff, but not everything is explained. Mysteries remain.

There is nowhere left to go but the overseer's office. Via a terminal, we open the sacrificial chamber. The overseer's desk raises from the ground to reveal a stairwell that goes down. There is a trail of human blood and organs. There is a door. Behind it is a long corridor. At the end of the corridor is a light. There is light at the end of this tunnel, light that was seen by scores of overseers as they walked to their death.

And now you're following in their footsteps.

"Congratulations, martyr!" declares a chirpy male voice from some hidden speaker. "Your fantastic journey is only just beginning. Please proceed to the light."

We proceed to the light.

"The light is calming, and puts your mind at ease. Go to the light."

We go to the light.

Through a door we see four glaring construction lights in a room, then another door that leads to another room. In it is a chair sat next to a projector that faces a screen.

"Welcome. Please sit in the chair. The show is about to begin."

...

"The show requires that you sit in the chair."

...

"It is absolutely essential that you sit in the chair."

...

"You have no other choice. You must sit in the chair."

We sit in the ruined chair. The screen flickers into life. It is a presentation by Vault-Tec, the evil company that built Fallout's vaults, ostensibly life-saving chambers, in truth horrific social experiments - and Vault 11 is one of the worst.

The presentation, which includes voice over from the chirpy male voice from before, is a filmstrip in the iconic Fallout loading screen fashion. The chirpy male voice tells us our sacrifice means the vault can continue to thrive. We see a picture of a man on a beach, sitting on a chair, cocktail in hand, watching the sun go down. This is a two-minute film about our lives, but it has nothing to do with real life. This is the unobtainable, perfect 50s American life imagined by Vault-Tec. This is Vault-Tec at its most monstrous, trying to convince its latest victim - the player - to accept their death peacefully, as if they should be grateful. This is also Fallout at its monstrous best - unsettling, insane and kind of funny.

The lights go off and the walls of the room raise to reveal rock hard robots and ceiling-mounted turrets who immediately attack. These robots are not messing about, and take everything you've got - all your stimpacks, all your performance enhancing drugs and all the ammo you're packing for your most damaging weapon. You survive the encounter by the skin of your teeth, panting, confused and horrified. What just happened?

Vault 11 is also deeply troubling. Its end seems inspired by the Milgram experiment on obedience to authority figures, an experiment that involved working out to what extent people would violate their own morals when ordered to. Vault 11 teaches us people would be willing to go quite far in such a situation. In a wonderful, unexpected twist, we find ourselves walking in the overseer's shoes, somehow a part of the deadly experiment as a player in-game and out. We are the guinea pig, and we find that we obey. We sit in the chair and wait to die, just like all the overseers did before us. This, I think, is Vault 11's true genius.

The robots still smouldering, we notice the floor is littered with over a dozen human corpses, each an overseer sacrificed for the good of the vault because the people voted for them to die. We count 16 corpses, which means Vault 11's social experiment lasted for 16 years after the bombs fell. New Vegas is set around 204 years since the Great War of 2077. We arrive at the scene some 188 years after Vault 11 fell into chaos, after the armed rebellion swept death throughout the place. Everyone died - or so it seems.

Behind another door is the vault mainframe. This was the mainframe programmed to kill the entire vault unless it was given a yearly sacrifice. Inside a terminal we find a recording named Vault 11 Solution.

"All right, I know you can hear me, so listen up," says an unnamed man. "There's five of us left. Five. Out of... I don't know how many.

"So, it's over. We've talked and it's over. We're not going to send anybody to die anymore. So shut off our water or gas us or do whatever it is you're programmed to do. But we're done listening to you."

This man was one of the five people we heard in the voice recording we found at the entrance to Vault 11 - and it sounds like the man who tried to stop the suicide, the man who, we suspect, escaped.

There's something else, an "automated solution response", which the five survivors would have heard after making the decision to defy the mainframe.

"Congratulations, citizens of Vault 11!" declares the same voice from the room with the chair. "You have made the decision not to sacrifice one of your own.

"You can walk with your head held high knowing that your commitment to human life is a shining example to us all.

"And to make that feeling of pride even sweeter, I have some exciting news.

"Despite what you were led to believe, the population of Vault 11 is not going to be exterminated for its disobedience.

"Instead, the mechanism to open the main vault door has now been enabled, and you can come and go at your leisure.

"But not so fast! Be sure to check with your overseer to find out if it's safe to leave. Here at Vault-Tec, your safety is our number one priority."

Vault 11, we now realise, was yet another social experiment by Vault-Tec, which wanted to see how far vault dwellers would go under order of an authority figure. When the five survivors we hear in the recording found at the entrance to the vault learned of this, when they realised all of the backstabbing, the politics, the threats, the fighting, the stress, the terror, the rape and the murder were based on a lie, they were so ashamed of their actions that they felt there was no option but to commit suicide. Better that than the true story of Vault 11 ever come to light.

Maybe Voice 1 felt differently. Maybe he felt the true story of Vault 11 had to come to light, to educate, to help prevent something like this from ever happening again. Did the other four try to silence Voice 1 after he refused to commit suicide? Did Voice 1 kill them in self-defense? Did Voice 1 murder them in order to save himself? Or maybe the other four committed suicide, as it first appears when we enter the vault and view the scene, hundreds of years later. We simply don't have all the answers. All we know is Voice 1 was the sole survivor of the horror of Vault 11.

Everyone else died.

Vault 11 starts with a bang - four in fact - and it gets better from there. In the slow exploration of the long abandoned place we learn the story of its people and the events that led to the incident at the entrance. It is a rollercoaster ride light on combat - save the killing of the odd mutated rat and the cacophony of chaos at the climax - but heavy on story. The peaks are packed with intense anxiety that hurtle toward revelatory troughs. There are moments throughout, such as when you learn what Katherine Stone put herself through for her husband, and, well, pretty much everything that has to do with the sacrificial chamber, that live long in the memory. Vault 11 is, quite simply, Fallout at its very best, another example of a seemingly innocuous quest leading to something surprisingly intricate and entrancing. (Don't forget to pick up that differential pressure controller, by the way.)