Rob Morrison, head of Atlantis Engineering, finds he must put aside personal issues when a crisis unfolds at the Thames Barrier. His ex-wife Sam is Director of Operations, and his estranged father Leonard a professor of engineering who warns the structure is potentially ineffective. But the trio must work together to avert the devastation facing London. As an evacuation of the capital begins, the Barrier is overwhelmed, wreaking devastation.

Rob and Sam manage to contact Leonard, deciding the only remaining hope of draining London of water lies in making a risky return to the submerged machine room. The trio face a race against time to save the city, with the lives of millions of people at stake.