A bright autumn morning, London shines in all its glory - its architecture, its iconic river and the impressive edifice of the Thames Barrier gleaming in the sunlight. Meanwhile, in Scotland, a storm of hurricane proportions crashes into the coast, devastating the sleepy town of Wick. Mountainous tides barrel mercilessly along the North Coast of Britain, heading for London. However, the authorities remain skeptical that nature could deal any blow that London’s Thames Barrier, the largest fixed tidal defense in the world, could not withstand. Nevertheless the reality is that the storm and seasonal high seas will soon converge in the Thames causing a colossal surge, big enough to flood London.
Rob Morrison, (Robert Carlyle) head of Atlantis Engineering, is just back from holiday, to be called straight to the Thames Barrier for a maintenance check, which angers him. His ex-wife, Sam, (Jesslyn Gilsig) the Barrier’s Director of Operations, is the last person he wants to see. Meanwhile his estranged father, engineering genius Professor Leonard Morrison (Tom Courtenay) is desperately attempting to contact the Barrier high command with a warning about the potential ineffectiveness of the structure. Upon receiving the news that a colleague has drowned during the devastation at Wick, with soaring water levels and a unique set of weather conditions closing in, Rob realizes that his father’s worst predictions are coming true. Rob, Sam and Leonard must put aside their personal differences to mount an operation to save London from certain destruction.
Prior to a dramatic forecast of the devastation to come from crisis control centre, COBRA, headed by Patricia Nash, (Joanne Whalley), the Deputy Prime Minister (David Suchet) finally begins the evacuation of the capital. Is it too late? Reports indicate that transport systems throughout the capital will come to a standstill in four hours and anyone left behind will have no way out. As a pillar of authority, Nash faces difficult decisions, knowing that her own daughters are lost in the heart of the city.
Subsequently the unthinkable happens: the Barrier is overwhelmed by an immense and terrifying surge, water begins to flow into London at a rapidly terrifying rate and volume, causing panic in the Underground as the torrent cascades through the tunnels. Thousands of trapped passengers are drowned. The devastation continues as the city’s greatest landmarks -Tower Bridge, the London Eye and Big Ben- are all submerged. Eight million Londoners are about to become heroes or victims as they struggle to survive the biggest disaster Britain’s capital has ever faced.
Having narrowly escaped the Barrier with their lives, Rob, Sam and Leonard realise that to save London, they must return to its submerged machine room and implement a high-risk plan to drain the city of water. This race against time is daunting when the lives of millions of Londoners are in their hands. What happens next will determine whether the city survives or is wiped off the face of the earth.