Doomwatch are joined by Doctor Robson, sent to undertake tests on the new 'Sahara' spray which is apparently capable of destroying all plant-life on the planet. But when Quist receives a phone call from the ministry, he is given orders to immediately suspend both Robson and Toby without any explanation. Closer investigation leads Quist and his team to discover that a new computerised security system run by Commander Keeping and Barker has become far too powerful, and is making its own decisions about government personnel.

Quist is concerned by the fact that, approximately every one hundred years, certain extreme weather conditions arise which lead to an increase in the water level of the Thames - and just one inch higher would result in the whole of London flooding. Even more disturbing is the fact that this increase is imminent.