A sequel? Not quite. Instead - and this is weirdly appropriate, given the ground it covers - think of this as more of an augmentation.
"What we've been working on is a expansion pack for Enemy Unknown in the style of big Firaxis expansions like Gods and Kings," explains Ananda Gupta, who's been leading the XCOM team on the creation on Enemy Within - a project he's finally able to talk about. "So Enemy Unknown tells the story of the alien invasion and the Earth's defence. Enemy Within is the same invasion. You're starting from day one and then proceeding through the same arc. It's an overlay on the original. It's not the next episode, and it doesn't take place after the ending of the first game. It's the same storyline - but with lots, lots more stuff."
It's an expansion in terms of mechanics rather than narrative, in other words, which sounds like the perfect route to take for a game that keeps its players as close to its mechanical heart as XCOM does. Anyway, if you're still a bit disappointed that you won't get to see off an entirely new extra-terrestrial armada, Gupta and company have a pretty surefire way of making it up to you. Or - ha ha! - meching it up to you. Trust me: a few paragraphs from now, that line is going to be really witty.
Enemy Within's biggest twist on the XCOM formula lies with a central choice about your soldiers. As the narrative unfolds, you'll now be allowed to enhance them a little - so do you want to enhance them genetically or cybernetically?
Choose the former option, and you get to indulge in some splicing and dicing. I'm promised it will be pleasantly disgusting. "By researching alien autopsies, you can unlock genetic modifications and put your soldiers into a gene lab that will give them extra powers," explains Gupta. "Stuff like augmented leg muscles so you no longer need ladders and pipes to jump up onto roofs, or you can have a brain modification that reflects psi attacks back onto the caster of the psi attack. Or maybe you have super bone marrow that can heal you during battle without needing a medkit."
And if you choose the latter? If you choose the latter, you get mechs.
"We've added mechs," nods Gupta, with understandable pride. "We've added what we call mechanised exoskeletal cybersuits, in fact. These are giant robot suits. You put your soldiers through this horrifying cybernetic augmentation process, and then once they're in this cyborg state they can wear these huge combat skeletons.
"What's cool is that once you do those mech augmentations, they become a new class," he explains. "Your sniper or whoever will become a mech trooper, which is a new class with a new training tree straight from squaddie down to colonel. In addition, every time you upgrade a cybersuit, the suit itself has tactical systems that you can select. The mechs really have two families of abilities, then: the kind that the soldier has trained and then the kind that are on the mech. The mech has stuff like a flame thrower, it has a kinetic strike module, which is tech speak for a big fist that you use to pound things, and it has stuff like a proximity mine launcher, for the more subtle touch."
Just take a moment to savour the fact that Gupta leads such a fun-saturated life at Firaxis that he feels the need to clarify exactly why mechs are cool. While you're doing that, he can explain how mechs fit into the established game.
Pretty snugly, apparently. "These guys are not oversized units," Gupta adds. "They're definitely very big compared to regular soldiers, but they're still one-tile guys, kind of like the bigger aliens. They're definitely going to be a nice imposing presence on the battlefield, though. They have a whole new set of weapons like the giant mini-gun thing, and a rail gun."
You can go all in on mech stuff or all in on gene-splicing, or you can opt for a mix. Whatever you choose, Firaxis' focus with this expansion is clearly on the tactical side of the game. The only difference these soldier modifications bring to the base-building component is the addition of two new labs and a single, very low -cost new tech to power both. The really clever ideas plays out on the battlefield, both as you use your new toys and - rather brilliantly - collect the resource that pays for them - a resource called meld.
"We've added meld to a lot of our maps," says Gupta. "To abduction maps, to UFO maps. Meld is in these canisters that you have to recover during encounters, but the canisters have self-destruct timers on them.
"So we've refreshed our maps with this new gameplay layer. You have this thing that you really want to get, but you also have to play it safe and you must decide: is that meld canister worth the risk of running out to get it when it's only got three turns left on the self destruct timer? Could I go slow and go into overwatch and accept the chance that I might lose it, or do I make a mad dash for it?"
I'm a mad dasher by nature, as my squad would agree if any of them were still alive, so meld running fits with the way I already like to play XCOM. What's fascinating, though, is how meld's going to tempt everyone into being a little more reckless. In the middle of a pitched battle, you're essentially being asked to pull off an impromptu John Madden touchdown.
"It's true," admits Gupta. "We really felt players were having fun with the tactical game, but we felt like, especially on the harder difficulty levels, heavily conservative play was being rewarded. There's nothing wrong with heavily conservative play - that's fine - but we wanted to make it so that it wasn't a no-brainer. Also, because that kind of approach tends to take more time, we wanted to give players a reason to speed things up if they so desire. Meld recovery is really a good refreshing element for the game."
That's the heart of Enemy Within, then, but it's not the full story. There will be new enemies - Gupta's particularly excited about the mech's ET equivalent, tentatively titled the Mechtoid, which comes with advanced firepower and durability and the ability to conjure a psi-shield when mind-melding - and new gadgets, like a poison grenade, which needs no explanation, and a needle grenade, which has a huge blast radius but won't penetrate cover.
Oh, yes, and finally there are the maps - including a very welcome addition from the past. "We've added a whole load of absolutely new maps, including multiplayer maps," concludes Gupta. "So for example we have crashed UFOs in urban settings, not just in the wilderness. We have a bunch of different takes on different environments, and I think the fans will be very happy. One of the things they've requested the most is more maps, more maps, more environments, more maps." He laughs. "We even made a farm map. That was a nice bookend. That very first video from the first vertical slice of Enemy Unknown was a farm, We weren't able to do a farm at the time. Now we have one. Now we have an XCOM farm map again."