The Happening is a departure for director M. Night Shyamalan: he abandons his trademark conceit of the twist ending to tell a straight-forward tale of horror. It's like going to a Gallagher show where he refuses to smash watermelons with a giant mallet. The only difference is that Gallagher's comedy is grim and depressing and The Happening is hilarious.
Yes, the plants of the Northeastern United States are fed up with how we've been treating them and decide to simultaneously release a toxin that causes humanity to commit suicide in various comical ways. Evidently this is something that is entirely scientifically valid, because a hot-dog obsessed lunatic says so at one point in time during the movie. Mark Wahlberg baffles as a Lemon Drink-eyeing science teacher and the part of Zooey Deschanel is ably played by a Tarsier. Mike, Kevin and Bill lend intentionally funny riffs to this masterpiece of unintentional comedy. (Unless there's some sort of real-life Happening, in which case they pre-emptively announce their allegiance to the plants.)