It's not beyond the realm of comprehension, of course, that the plants would fight back, that after centuries of abuse by man, vegetation would simply turn its back on us. For example: Just try to buy a tomato this week.
But where Night Shyamalan goes wrong with The Happening is by purporting that it would somehow be a coordinated effort by all plant life, and that its revenge against mankind would involve a deadly airborne toxin emanating from plants that causes anyone it infects to kill themselves in a brutal fashion.
That's just silly.
And unfortunately for Shyamalan, that's all he's got.
The Happening never begins to make more sense than it does when a woman stabs herself in the neck in the film's opening scene or when construction workers leap to their deaths from the tops of buildings in the next scene. We know that something's happening and that our heroes, Mark Wahlberg and Zooey Deschanel, will probably outrun it. Anything else – what the real cause is, why it starts happening, etc. – is not really Shyamalan's concern.
Once upon a time, this guy made effective thrillers. If you like Signs, you could argue he made three straight effective thrillers to that point, following The Sixth Sense and Unbreakable. If that's your argument, the scales are now balanced: The Happening is the third bad Shyamalan movie in a row, succeeding The Village and Lady in the Water, and this is the worst of the three.
The acting is uniformly bad. Compare this with last year's The Mist, which, while not a great movie, at least had the proper amount of panic in each scene. The characters in that film didn't know what to do and their tension was palpable. That isn't the case in this rather nonchalant horror flick, which never heightens the anxiety, is paced poorly, and to make matters worse, throws in comic relief where it isn't needed or even wanted.
Beyond that, the idea is not fully formed and the resolution is entirely too weak. There should be more of a reason to watch Wahlberg and Deschanel amble through the Pennsylvania countryside with no clear destination in mind. Perhaps Shyamalan should've made that point to himself while writing the script, which starts with the concept of man vs. nature – one of the five basic conflicts of dramatic storytelling – and never goes any further.