After NOVA yesterday, we went to see Signs. If you are willing to suspend a certain amount of disbelief, it is a suspenseful and entertaining film. I was quite pleased at the level of humor mixed in with the creepiness. The final five or ten minutes left me feeling cheated though. WARNING: Spoilers in the next paragraph!
It seems to me that Shyamalan's never read any science fiction. At least, not in the Golden Age. And he seems to have little knowledge of comic book cliches, for somebody who did a critical reinvention of the comic book superhero. [Okay, spoilers ahoy!]
So aliens come to Earth in vessels which can hover in place, erect invisible shields, silently construct crop circles, and so on. The individual aliens can outrace an athelete, jump ten feet to the top of a roof, and have spurs which emit poisonous gas. Yet when they finally choose to engage, they attack hand-to-hand, and are defeated by water? What kind of incompetent boobs are they anyway? Can't they wear raincoats when they attack?
Then it occurred to me. This was not an alien invasion at all. It was hunting season. Earth supplies an animal which can potentially put up a rather challenging struggle. The Intergalactic Gaming Commission sets the rules. You may only hunt on Earth once every XXXX years. You may not take advanced weaponry. You must stay within XX miles of the preserve markers (signs). You may not wear flak suits. Sorry buddy, that's the challenge. It's a water world, the inhabitants are 80% water, and you just happen to have a severe allergy to molecular water. Cry me a river. Do you want the permit or not?
So there you go. The only logical explanation for the aliens' incompetent approach to invading Earth, and why they conveniently bailed after one night of actual aggression. They weren't actually 'defeated' by the 'primitive defense'. The hunting season was just one night. Everything leading up to it was just the tailgate party!