A planet has been found with a near-eternal supply of fresh water that barely needs to be filtered. There is a race of merpeople living on the planet. The planet has been nicknamed Salvation by the humans, who take it over to supply fresh water to the human colonies. The merpeople are forced to “relocate” to a massive underwater walled city called Mariana by the humans but nicknamed The Trench by everyone. The Trench is an underwater Kowloon city; catastrophically overpopulated and ridden with crime, disease and poverty.
The human government brands every merperson at the age of one, because the mers can morph their tails into legs to walk on land for short periods of time. Their gills are in the backs of their throats so it’s almost impossible to distinguish them from humans. The brand is done by laser and is on the back of the neck, at the base of the skull. On branding day, every mer with a one year old child is required to bring the child to the branding. Anesthesia is available for those who come voluntarily, but there are always those who try to hide their children. Human soldiers sweep the trench every branding day and root out the hidden children, whose parents are forced to watch as they’re branded without anesthetic. There are penalties for unbranded older children, adults who have escaped branding or tried to hide them and other offences - facial branding, air torture, fin cutting, labor and penal colonies, and children taken away from their parents. Mers disappear regularly to never be seen again…they are sent to the water harvesting plants or mining colonies where the life expectancy is very short.
The government also utilizes child labor in secret. Parents who have their children removed are told that they’re going to be sent to government run orphanages, but are instead sent to the labor colonies to do work that adults can’t to because they’re too big, like clearing blockages in pipes. Mortality rates are extremely high. An escaped child reveals this to the mers, which is the seed that plants the idea of rebellion.
Around the same time, a mer woman is caught topside after curfew and raped by human police. The government covers up the crime and the woman disappears, but there was a secret witness.
There are mer rebels who have found ways out of the Trench, even though the penalty for any mer caught outside is death by topside public “air drowning”…mers must submerge and morph every twelve hours or so to survive. The rebels are unbranded or have found ways to hide their brands to walk around topside undetected. They have infiltrated government to high levels and have a system for hiding children outside the Trench on branding days, census days, and the day they’re born to avoid detection. The high-level government rebels have sophisticated hidden water tanks in their houses so that they can live topside as humans.
The rebels discover a secret that the humans have been keeping for a number of years – that Salvation is running out of water. The human government is planning a genocide of the mers by draining Mariana to squeeze all the water they can out of the planet, which will be teraformed and colonized by humans once it’s nearly dry. The rebels hatch a plan to overthrow the human government and seize the teraforming equipment to rehydrate Salvation. In the process of planning the coup, they discover that the second highest ranking government official is a mer who leaked the genocide plans to the rebels. He helps them successfully overthrow the government at the cost of his own life, and Salvation is returned to its natural state. The mers start selling water to the human colonies.