Scientologists believe that 75 million years ago an evil galactic ruler, named Xenu, solved overpopulation by bringing trillions of people to Earth in DC-8 space planes, stacking them around volcanoes and nuking them. Then the souls of these dead space aliens were captured and boxed up and taken to cinemas where they were shown films of what life should be like, false ideas containing God, the devil and Christ and told to get ill.
After that they supposedly clustered together and now inhabit our bodies. Scientologists believe that if they rid themselves of these body thetans then they will be healthier and will gain special powers like mind-over-matter.