The movie begins with a team of scientists working on a US government project, known as Stormshield, whose goal is to control and manage storms, particularly hurricanes.
Hurricane Grace, a Category 3 hurricane which is slowly climbing to a Category 5, is up to hit the United States. Using a predictive technology named Tempest and developed by Lance Resznick, they are able to simulate the effects of seeding the storm in order to collapse the eye of the storm and then decrease its intensity.
Lance is openly skeptical towards the theories of Sara Hughes, an English scientist who is convinced about the effectiveness of cloud seeding. An experiment is done on a smaller hurricane, Agatha. A plane and several UAV carrying the seeds (in this case, supercooled liquid) fly into the storm. At first, the experiment is a success but then the storm intensifies during the seeding and the plane crashes.
Weeks go by and Grace has now become a category 5 hurricane, headed straight for Miami. At the urgings of Katzenberg, the fund seeker of the project, the team formulates a plan of distant atmospheric perturbation to deflect Grace out into the Atlantic Ocean by creating a low pressure system on the Pacific coast and allowing it to be carried across the nation towards the hurricane.
While the team leader Abrams and mathematician Munish Loomba try to model the weather perturbation that will safely deflect Grace, Lance tells Sara that her attempt to change Agatha’s course after the seeding was actually successful. Discussing such results with her grandfather, who was head of a similar, discredited project in the 1970s, Sara discovers that he and his team knew they could make hurricanes change course but made their efforts appear fruitless because they realized the military were behind the project, looking for ways to use weather as a weapon. The same turns out to be true for Stormshield, even if Katzenberg had previously assured that there was no military involvement.