Kar-Charratans were aquatic life forms who lived on Kar-Charrat that survived in liquid form, usually in water. They had the capability to inhabit life forms containing water, and were able to manipulate water to attack and drown other life forms. They were even capable of using water to communicate. Visitors to the planet who heard their voices often mistook them for phantoms.
Most Kar-Charratans were captured by Chief Librarian Elgin and placed inside the wetworks facility, where they were used to download data, a process which often destroyed their minds. When the Seventh Doctor and Ace arrived on the planet, they attempted to communicate with them, but failed. They then inhabited the body of the deceased Rappell to free Ace and Bev Tarrant, while the ones in the wetworks facility told the Doctor of their plight. The Doctor destroyed the facility, freeing the Kar-Charratans. (AUDIO: The Genocide Machine)