In the park in New York city people behaved like in a Flash mob, officially, the first known flash mob happened in 2003, in Manhattan, New York, and the movie was from 2008.
They were freezing, and some of them were walking backwards. Maybe some people knew, others don't.
In New York City's Central Park, people begin committing mass suicide. The event is believed to be caused by a bio-terrorist attack using an airborne neurotoxin. The behavior quickly spreads across the Northeastern United States.
High school science teacher Elliot Moore and his wife Alma are persuaded by Elliot's mathematician colleague Julian to accompany him and his daughter Jess on a train out of Philadelphia. During the trip, the group learns that Boston and Philadelphia have been affected. The train loses all radio contact and stops at a small town.
When Julian learns that his wife has left Boston for Princeton, he decides to look for her and entrusts Jess to the Moores. However, Julian arrives to find Princeton has been affected, causing the driver of the car in which he is riding to ram into a tree. He survives but commits suicide by slitting his wrist with a glass shard.
Elliot, Alma, and Jess hitch a ride with a nurseryman and his wife. The nurseryman hypothesizes that plant life has developed a defense mechanism against humans consisting of an airborne toxin that stimulates neurotransmitters and causes humans to kill themselves.
The group is later joined by other survivors coming from various directions, and the small crowd chooses to avoid roads and populated areas. When the larger part of the group is affected by the toxin, Elliot suggests the nurseryman was right and that the plants are targeting only large groups of people. He splits their group into smaller pockets and they walk along.
The trio ends up with a pair of teenage boys, Josh and Jared, who are later shot and killed by the armed residents of a barricaded house. Elliot, Alma, and Jess wander the countryside and come upon the home of Mrs. Jones, an eccentric and paranoid elderly woman. Jones initially agrees to house the group for the night but is suspicious of them having bad intentions; the next morning, she decides to expel them.
In a fury, she leaves the house alone and is affected by the toxin. The shaken Elliot realizes that the plants are now targeting individuals. Left with no option when Mrs. Jones strikes her head into several windows, the trio chooses to die and embraces in the yard only to find themselves unaffected by the toxin.
The outbreak has abated as quickly as it began. Three months later, Elliot and Alma have adjusted to their new life with Jess as their adopted daughter.
Alma learns she is pregnant and surprises Elliot with the news. On television, an expert compares the natural event to a red tide and warns that the epidemic may have only been a harbinger of an impending global disaster. In Paris's Tuileries Gardens, people begin committing mass suicide.
