When a hurricane-force storm destroys a Scottish coastal town and causes a chain reaction of huge tides along the east coast of Britain, authorities remain sceptical that the Thames Barrier is in any danger of being breached. However, Professor Leonard Morrison and his engineer son discover a potential flaw in the barrier's structure, and attempt to mount an operation which could save London from certain destruction.

As millions of Londoners fight for survival in the devastated capital, Rob, Sam and Leonard come up with a high-risk plan to drain the city of water. Their actions are crucial to the entire city's survival, but will they manage to implement their plan in time to save lives?