Jo Harding (Helen Hunt) became a storm chaser (a meteorologist who photographs and scientifically studies tornadoes in the field) after her father was sucked into a large twister and killed when she was a girl.
Jo and her husband, Bill Harding (Bill Paxton), also a storm chaser, invented DOROTHY, a gizmo designed to release thousands of tiny sensors when a tornado passes over it. The Hardings hope the information transmitted by the sensors will provide insight into the nature of the whirling windstorms. However, when Bill left to become a successful weatherman, their marriage suffered and they separated.
Meanwhile, Dr. Jonas Miller (Cary Elwes), who is backed by a large corporation, has created a similar machine. Neither gadget has been field tested and both groups of storm chasers are anxious to find tornadoes. At the peak of the worst twister season in decades, Bill shows up with his fiancée Melissa (Jami Gertz) so Jo can sign the divorce papers. When a twister is suddenly spotted, Jo rushes off after it. Bill joins the mad rush to reach it in time to activate DOROTHY.
Throughout the day the storms become worse and the Hardings race against Miller to test their machine first. As they face danger together, Jo and Bill realize they still have feelings for each other.