In the 2005 film Serenity, the secret about the Reavers is discovered on the planet Miranda.
Miranda was intended by the Alliance to become a model colony, but the experiment went disastrously wrong. To control the population, Alliance scientists released a behavioral chemical called G-23 Paxilon Hydrochlorate, commonly shortened to Pax. It suppressed aggression and motivation so severely that most inhabitants stopped eating, working, or even moving.
A small minority reacted differently: the chemical amplified violent impulses rather than calming them. These survivors became the Reavers, savage and territorial people who spread beyond the Alliance’s controlled space. The government concealed the disaster and claimed Miranda had been abandoned for an ordinary reason.
Malcolm Reynolds and the crew of Serenity reach Miranda after following clues from River Tam and the hacker Mr. Universe. They find the population dead and record evidence of the Alliance’s experiment. Mal later forces the Operative to watch the footage, exposing the truth behind the Reavers.