On the "advanced" levels (called OT levels) above the state of "Clear" they encounter the story of Xenu. Xenu was supposed to have gathered up all the overpopulation in this sector of the galaxy, brought them to Earth and then exterminated them using hydrogen bombs. The souls of these murdered people are then supposed to infest the body of everyone. They are called "body thetans". On the advanced levels of Scientology a person "audits out" these body thetans telepathically by getting them to re-experience their being exterminated by hydrogen bombs. So people on these levels assume all their bad thoughts and faulty memories are due to these body thetans infesting every part of their body and influencing them mentally. Many Scientologists go raving mad at this point if they have not done so already.
