In the sunken kingdom of Atlantis, food is a precious commodity. It is hard to gather, and once taken from the sea, it spoils within hours. To solve their problem, the Atlanteans decided they needed to make use of farmers. When they came across survivors from shipwrecks, who otherwise would have been corpses, they force them to undergo an operation in which they are given plastic gills. Thus able to breathe underwater, these "fish people" are sent into the sea to gather plankton for the Atlanteans. Their desperate need for food means that they also need all the human labor they can get.
Some Fish People are very human in appearance, with only goggles and a bathing cap marking them apart from ordinary people. But other Fish People have become quite aquatic-looking, with large fishy eyes, fin-like ear appendages and shiny scales covering their bodies. (The suggestion is that they may gradually become more fish-like the longer they spend underwater, much like haemovores.) As they also retain their human breathing organs, they can breathe above water just as easily as beneath it. They still understand English (or, presumably, whichever language was their native one) but they communicate with one another using bubbling noises.
In effect, the Fish People are nothing more than slaves. They are forced to work for the Atlanteans without getting anything in return. Unfortunately, perhaps as a side-effect of the operation, they are not very strong-willed people. Their most rebellious acts seem to be throwing rocks and seashells at people who taunt them. But when the Doctor needs to throw a spanner into the works of Atlantis, to stop the evil plans of Professor Zaroff, he suggests that the Fish People could go on strike, and refuse to collect any more food, thus forcing the Atlanteans to agree to their demands or starve. When this is suggested to the Fish People, they agree to the plan, and word soon spreads of their revolt. When the Atlanteans learn that no more food is being gathered, they begin to panic, and the Doctor is able to overthrow Zaroff's plans. The lower levels of Atlantis are flooded to prevent anyone else using his equipment, and the Atlanteans plan to rebuild their city...without using fish people for slaves. Presumably, the Fish People are henceforth free from their previous slavery.
