This is one of those few true greats in freeware games. At first glance, it's a standard platformer. Go a bit deeper, though, and you'll find a complex storyline, impressive pixel animation, and at times shockingly difficult without being impossible.
Some games are hard because they're poorly designed; this one is hard because it's EXCELLENTLY designed.
There's jumping, shooting, multiple weapons and upgrades, uniquely designed levels, and some fantastic bosses. Most anyone who's played it will tell you that it's one of the best games out there. Give it a try, and see if you agree.