No plot whatsoever... there was an attempt at one, but they totally failed to actually make it work. It was basically a series of shots of people getting killed in various gruesome ways, along with Samuel Jackson running around with a makeshift flamethrower.
Oh yeah ... you'd think with a name like "Snakes on a Plane", there would've been at least some humor. There wasn't.
It's a fine movie, if you just want to see a bunch of carnage and dead people. But otherwise, it's utterly worthless.