The film opens in New York. People start to get confused in Central Park, repeating their words, standing still and sometimes walking backwards. We hear a few screams. A woman reading on a bench takes her silver chopstick-style hair pin out of her hair and stabs herself in the neck with it.
The plant's chemical defenses were the culprit. That insight seems based on 'New Study Sheds Light on Plants’ Nighttime Defense'.