You're a military soldier. And the gang (Aztecs) Took the KACC and stole an Helicopter, wich is from area 51, You gotta recover the KACC and the stolen helicopter with help of other soldiers.
It's pretty long, and it's hard if you don't take cover.
It's my first mission and it's probably a little messed up, But i put all my effort into it.

Oh, And, At the end of the mission, When you enter Area 51 with the Hunter helicopter, Those thing (The ones that launch rockets) will shoot you rockets, Don't worry, the helicopter is invencible to explosions.