It's kinda a weird series. 3-5 have been amusement park-style open world first person shooters, where you have to liberate local people from oppression by sneakily pressing F near enemies to unleash mostly-unexplained knife-jitsu, despite being some American kid a "Far Cry" from home (a different kid each time). They're kinda stealth-action shooters which threw realism out the window. They also have long trippy cutscenes, often involving drug use, and casts of quirky outlandish characters both on your side and the enemy. There's also magic? Probably? The drugs make it hard to tell.
Far Cry 2 was kinda like that, except there's no humor. You're a mercenary in Africa who has to earn the trust of *both* factions in a civil war, by killing people on both sides, so you can eventually kill the protagonist of the first game. It's absurdly gritty. Guns break, people try to drag their wounded friends away from you, the only currency is blood diamonds... The only light comes in the form of your fellow mercenaries, who are your bestest friends and nothing bad happens.
(If it isn't clear, it's one of my favorite games of all time)
I never played Far Cry 1 but I think there were aliens or something.