There's a moment, very early on during my time with The Bureau: XCOM Declassified , when I realise that I'm definitely playing an XCOM game. And funnily enough, it's not the point at which I slow down time and order a complex series of flanking manoeuvres followed by special attacks to my two companions. It comes immediately afterwards, when I arrogantly decide that, actually, I've played tons of third-person shooters, thanks very much – and all this 'tactical' nonsense is a bit beneath me and my awesome headshot abilities. Sectoids have oversized skulls anyway, right? Shouldn't be too hard to nab that damage bonus. And so, I decide to run-and-gun my way through the next wave of alien forces. They get my squad members first of all. Without the direction of their commander, they suddenly seem overwhelmed by the stresses and strains of extraterrestrial combat. And then, when the aliens have finished with my teammates, they come for poor outnumbered, outgunned me. It's a dismal, humiliating failure but, as protagonist William Carter slumps to the ground after a failed attempt to resuscitate one of his companions, I'm filled with a powerful sense of joy.

I'm relieved because 2K have done the... not quite impossible, but certainly downright tricky. They've managed to make a game that preserves the spirit of XCOM even as it abandons many of the core tenets of the series. Tenets like its genre, for instance. Making a shooter out of the strategic series always seemed a strange idea (at least, it did from a creative standpoint; from a business standpoint it was drearily on the money). The rigid lines of a grid-based arena; the discrete, binary nature of turn-based decision making; the inability of the player's own twitchy reflexes to tilt the odds in their favour – all of these aspects seemed essential to crystallising the tactical nature of the game. And that nature seemed in danger of being diluted by real-time combat, in which you don't have to watch the consequences of your mistakes play out as you sit, agonisingly impotent, during the enemies' turn.

But The Bureau works because it's hard. It works because these aliens aren't the cowardly, cover-skulking critters you'll recognise from other third-person shooters, but an aggressively punishing, coordinated force. And it works because you still need to micromanage your squad of three proto-XCOM operatives to defeat them. Inevitably, it does feel different to Enemy Unknown. It's inescapably more of an action game. But I expect the two titles would find plenty to talk about (in hushed conspiratorial tones, naturally) were you to introduce them at a party.

The Bureau is a prequel to Enemy Unknown, chronicling the founding of the organisation. But not so long ago, this rather conceited spin-off thought itself a reboot – a first-person take on XCOM that you'll dimly remember if you cast your mind all the way back to E3 2010. XCOM (as it was known back then) looked, well, not very much like XCOM at all, given its theme of blasting hunks of animate black goo in the suburbs of '50s America. And then, like a lone hitchhiker who'd spotted some mysterious lights floating in the sky, XCOM vanished, only to reappear following some rather invasive probing.

Last summer's version of XCOM was a tactical FPS in which the player could temporarily pause the action and issue orders to their companions. 2012 was only big enough for one XCOM game, however – and that turned out to be Firaxis' rather excellent Enemy Unknown.

While gamers were distractedly praying that sniper shots wouldn't miss, and trawling their Facebook friend lists for soldier names, 2K quietly took XCOMthe- shooter away for one final retool. With that accomplished, The Bureau: XCOM Declassified is now a third-person shooter with a heavy influence on squad management. So, I ask Alyssa Finley, vice president for product development at 2K Marin, why so many overhauls?