Ben's on a trip to London to meet his mum. But an accident at the Thames Barrier, combined with a tidal surge and a dramatic thunderstorm - and suddenly his trip turns into something totally different as the Barrier is breached and London is flooded. With streets underwater, communications down, rats pouring up out of the sewers and thousands of people in a state of panic, survival becomes a key issue. But as Ben tries to get across London to meet his mother, little does he know that two terrorists have a similar rendezvous ...