For millennia, Egypt has depended upon the waters of the Nile. Its annual floods fertilize the land. By the time Britain extends its dominion over Egypt, the Cairo Barrage is the key to control, its name taken from the French term meaning a dam or irrigation channel designed to increase a river's depth or to divert its flow.

An attempt to blow up the Manufiyah regulator in the Barrage is not a petty matter. Gareth Owen, The Mamur Zapt, Chief of Cairo's Secret Police, is hurriedly summoned. He hasn't a clue what a regulator is. Nor can he identify the mysterious Lizard Man who seems to have a grudge against the whole Egyptian irrigation system. But he does know that the ceremonial cutting of the temporary dam thrown up each year across the mouth of the Khalig Canal restarts the whole irrigation cycle, allowing water to pour through the canal and signal the opening of dams all across the land. The cut will require policing. Especially as it is going to be the last cut before the canal is filled in.

While this modernization will wipe out a health hazard, the Cairenes love the party that goes with the cut. They are less than happy with the dam. Things grow worse when a young woman's body is discovered at the site. Owen has to say it's extremely embarrassing. Is this the traditional ritual sacrifice? Or a sign of sabotage? A diversion? Or just plain murder?