In Scientology, a thetan is the term corresponding to one's "spirit", and a Body Thetan is another thetan (spirit) who is 'stuck' in, on or near a human "meat body" because of an incident caused by an ancient galactic dictator named Xenu. All human bodies were said by L. Ron Hubbard to be covered in clusters of these misplaced thetans.
This information was originally published openly by Hubbard and the Church of Scientology in a number of "policy bulletins" (HCOBs) and "policy letters" (HCOPLs), but were later made secret. It was also alluded to (although not described in any detail) in a publicly available cassette tape Hubbard released called "Ron's Journal 67", addressed to "all staff and students of Scientology organizations". For some years, it was reported by former Scientologists that the material was still being used, but not divulged until one reaches the third Operating Thetan level (or OT III).
Free Will 
According to Hubbard, body thetans cling to a body because they have lost their free will as a result of an event in their past. There are several Scientology Auditing 'processes' which are believed to help a body thetan restore free will. Upon reaching OT III, the individual finds body thetans by locating any sensation of pressure or mass in his or her body. This is addressed telepathically as a cluster, and taken through the cluster-making incident of 75 million years ago.
Often members of the Church of Scientology will publicly deny the existence of space opera doctrines, or attempt to minimize their importance. Because the secret information imparted to members is to be kept secret from others who have not attained that level, the member must publicly deny its existence when asked. OT III recipients must sign a waiver promising never to reveal its secrets before they are given the manila envelope containing the Body Thetan knowledge. It is supposedly knowledge so dangerous, as noted on the "Ron's Journal 67" cassette, that anyone learning this material before he is ready could die though many have learned the story and remain alive.
Despite the Church's efforts to keep the story secret, details have been leaked over the years. OT III was first revealed in Robert Kaufman's 1972 book Inside Scientology: Or How I Found Scientology and Became Super Human, in which Kaufman detailed his own experiences of OT III. It was later described in a 1981 Clearwater Sun article by Richard Leiby, and came to greater public fame in a 1985 court case brought against the Church by Lawrence A. Wollersheim. The Church attempted to keep the case file checked out by a reader at all times, but the story was synopsised in the Los Angeles Times, November 5, 1985 and detailed in William Poundstone's Bigger Secrets (1986) from information presented in the Wollersheim case. Church lawyer Warren McShane later claimed the story had never been secret, although maintaining there were nevertheless trade secrets contained in OT III. Notably, McShane discussed the details of the Xenu/Body Thetans story at some length and specifically attributed the authorship of the story to Hubbard. Audio recordings exist of Hubbard lectures that discuss Body Thetans and other Space Opera subjects.
See also 
- "What is Scientology: OT III Released". A description of the OT III released. Church of Scientology.
- OT III Course, summary and comments