A powerful storm surge strikes the United Kingdom, coinciding dangerously with the spring tide and leading to significant devastation in parts of Scotland, particularly Wick. The head forecaster at the Met Office, Keith Hopkins, miscalculates the storm's trajectory, believing it will head towards the Netherlands. However, marine physics professor Leonard Morrison refutes this theory through his own thorough and advanced computer model. His analysis indicates that the storm and the tide could converge, generating a towering surge of 46 meters.
As panic ensues due to a series of mistaken communications with Deputy Prime Minister Campbell—who grows agitated over the inaccurate forecasts—Hopkins grapples with his guilt for wasting precious time. In a critical meeting at COBRA (Cabinet Office Briefing Rooms) in Whitehall, authorities acknowledge that Central London faces an impending inundation within a mere three hours. With the declaration of a state of emergency, chaos erupts throughout the city as residents scramble to evacuate.
Designated as the emergency coordinator by Campbell, Police Commissioner Patricia Nash collaborates closely with Major General Ashcroft, transport chief Johnson, and the remorseful Hopkins, all tasked with the enormous challenge of evacuating over a million people from the perilous city.
In a desperate turn of events, Leonard confronts his estranged son Rob, an engineer working on the Thames barrier, and Rob's ex-wife Sam, who manages the barrier itself. As Sam exercises her authority over raising the barrier, disaster unfolds when the tsunami arrives, overwhelming their defenses and flooding the city.
Rob and Sam make a harrowing descent into the Thames to flee the flooded barrier while Leonard is airlifted by a military helicopter to Whitehall to provide insights to COBRA. In the London Underground, Rob and Sam encounter two maintenance workers, Bill and Zack, but tragedy strikes when Bill sacrifices himself to save others as the flood engulfs the station.
After a narrow escape to the surface, Rob and Sam find themselves in a submerged Trafalgar Square, where they manage to reach out to Leonard. Upon reuniting, Leonard unveils his belief that reversing the water flow is possible since the tide has shifted and water levels begin to fall. However, General Ashcroft opposes this notion, urging the Prime Minister to greenlight an airstrike meant to destroy the barrier in hopes of creating a surge that could expel the floodwaters from London.
Haunted by his role in the catastrophe, Hopkins succumbs to despair and takes his own life. Meanwhile, Nash and Ashcroft clash over giving the Morrisons a fighting chance to rectify the crisis. In an emotional showdown, Rob, Sam, and Leonard learn that the controls necessary to reverse the water flow are now submerged underwater. As they deliberate on who should attempt the daring activation, Leonard makes the profound choice on their behalf, stepping forward to undertake the perilous task himself.
With stalwart determination, Leonard succeeds in activating the controls but tragically drowns in the process. Just as the airstrike is called off and the water recedes, Rob finds solace in Sam's arms as Leonard's body is recovered from the scene. This disaster has claimed over 200,000 lives, and amidst the profound loss, Campbell addresses the city's survivors via a radio broadcast, reassuring them that London will endure and begin to rebuild.
