An anomaly opens up at the British Museum in the middle of an ancient Egyptian artefact called the Sun Cage. Through this comes a huge crocodilian creature that kills a museum worker, Marion. Sarah Page has an encounter with this creature and relates it to representations of the Ancient Egyptian demon, Ammut. Connor establishes that it is in fact a form of ancient crocodile called a Pristichampsus – heavily armoured and with long running legs making it very fast. The team track the creature across London on its way to the Thames River, eating an unsuspecting traffic warden along the way. The creature was injured and returned to its home era through the anomaly. Nick Cutter realised that the Pristichampus had gone through the anomaly in Egyptian times and therefore they based Ammut on them.