Kadan was a Quarren doctor who used drugs to control people who were kidnapped and used for the Quarren workforce. He made false diagnoses on people, altered their memories so he could solve the work shortage on his planet. Almost all of the USS Voyager crew was subject to this treatment.
Chakotay, who was on an away mission when the crew was kidnapped, returned to the plant and attempted to expose the plot. Kadan had him arrested and altered his memories also. Seven of Nine who was brainwashed, became suspicious and accused him of the mind altering. Kadan was enraged that one of his associates, Dr. Ravoc, allowed Seven to access files. Ravoc confronted Kadan, and he admitted what he had done. Kadan claimed it was to cure the labor shortage and improve the patients' lives, making them productive people. Kadan was also paid a large sum of money to do this. He said that the Quarren Ministry of Health was behind him and that there were those in Law Enforcement that were involved. Ravoc threatened to report him, and Kadan tried to alter his memories.
After a police officer named Yerid was convinced by Chakotay of the plot, he helped free the Voyager crew and arrested Kadan, exposing the conspiracy to the authorities. (VOY: "Workforce", "Workforce, Part II")