the movie is about plants that begin releasing neurotoxin defense systems in the air to fight back against humanity's pollution efforts. the toxin causes people to lose their 'survival instinct,' which apparently means they just immediately try to kill themselves. wahlberg and zooey are an estranged husband and wife who spend the entire movie on the run with one another, hating each other and loving each other along the way. it's emphasized that he loves her, and she hates him, but might still have feelings.
the plants 'attack' the big cities first, since they're the biggest polluters. then the highways, then middle sized towns, etc. there is a discussion about how the plants have 'triggers' that make them go off and 'attack' intruders. the longer they attack, the more sensitive their triggers get. soon, it gets to the point where they release the toxin in defense against groups as small as two or three people in large, rural areas (the places they were fleeing to to get away from places they thought would be attacked).
trapped in a house after seeing a single woman killed by the neurotoxins, in some off-the-grid corner of nowhere, wahlberg and zooey realize they have no hope of going outside again. suddenly, for no reason, and out of nowhere, wahlberg realizes the woman wasn't killed for being a threat to the plants, but because the final trigger was 'human energy.' the woman who was killed was angry at the world, and harbored negative energy. wahlberg and zooey realized they had to reflect on their past relationship to remember the days when they loved each other, in order to harbor enough positive energy to survive outside. they daringly take a step outside, and embrace as they realize the attack is over.
LAWLZ FOR FUCKING MILES.
also, there are dozens of times throughout the script where he notes that the scene should be shot from the "point of view of the plants" as they stare out menacingly at the people, as we are unsure if they are about to attack.
furthermore, there are two recurrent themes foreshadowing every attack:
1) a date/time stamp with the location appears on screen. this means shit is about to go down and the plants are about to attack.
2) a breeze starts to pick up, to disperse the neurotoxin. apparently the plants in this movie are working in conjunction with the wind to summon the breeze at will to incur their wrath. every time the wind kicks up, or you see plants swaying in the breeze, you better RUN...because people can clearly outrun wind.
oh, and as if it even needs to be said, the movie was heavy handed as fuck in its environmental message. characters might as well have turned around towards the camera and say LOOK AT WHAT WILL BECOME OF US IF WE CONTINUE DOWN THIS PATH.