XCOM: Enemy Within isn't a full-fledged sequel to Firaxis' 2012 masterpiece: it's more of an expansion. However, that qualifier alone generally tends to automatically lower the expectations of the fans. But that shouldn't be the case here. This release is bursting with quality content that differentiates itself far more than it needs to in order to escape allegations of "it's the same game." It feels different in some ways, and thankfully, those differences are meaningful.
Enemy Within introduces a rogue faction known as EXALT. They, like XCOM, are a paramilitary force that deals with supernatural threats. However, unlike XCOM, they understand the motives of the alien race that invaded under the orders of the Uber Ethereal, and wish to advance the human race beyond what it currently is. This in and of itself may seem like it could be viewed as a worthwhile cause, but unfortunately, they also want to take over the planet. So they do so however they can, whether it's on the field of battle or on the global front, spreading panic and pursuing a ruthless cyberwarfare campaign with XCOM itself.
