The following portion of OT 3 is quoted in full in Margery Wakefield's book The Road to Xenu , chapter 13, and I believe all of it has been quoted in other books and articles as well.

The head of the Galactic Federation (76 planets around larger stars visible from here) (founded 95,000,000 years ago, very space opera) solved overpopulation (250 billion or so per planet - 178 billion on average) by mass implanting.   

He caused people to be brought to Teegeeack (Earth) and put an H-Bomb on the principal volcanos (incident II) and then the Pacific area ones were taken - in boxes to Hawaii and the Atlantic area ones to Las Palmas and there "packaged". His name was Xenu. He used renegades. Various misleading data by means of circuits etc was placed in the unplants. When through with his crime loyal officers (to the people) captured him after six years of battle and put him in an electronic mountain trap where he still is. "They" are gone. The place (Confederation) has since been a desert.

The length and brutality of it all was such that this Confederation never recovered. The implant is calculated to kill (by pneumonia etc) anyone who attempts to solve it. This liability has been dispensed with by my tech development. One can freewheel through the implant and die unless it is approached as precisely outlined. The "freewheel" (auto-running on and on) lasts too long, denies sleep etc and one dies. So be careful to do only Incidents I and II as given and not plow around and fail to complete one thetan at a time.

In December 1967 1 know someone had to take the plunge. I did and emerged very knocked out, but alive. Probably the only one ever to do so in 75,000,000 years. I have all the data now, but only that given here is needful.                                                     

One's body is a mass of individual thetans stuck to oneself or to the body.                                                     

One has to clean them off by running incident II and Incident I. It is a long job, requiring care, patience and good auditing.                                                     

You are running beings. They respond like any preclear. Some large, some small.

Thetans believed they were one. This is the primary error.

Good luck.

After Hubbard had made these remarkable "discoveries", Scientologists - the majority of whom were science fiction fans, not at all coincidentally - also began to "recall" Invader Forces and implants. This allowed the following definitions to be made (taken from the Dianetics & Scientology Technical Dictionary, 1975):

INVADER FORCES

     1. an electronics people. The electronics people usually happen to be an evolutionary line which is on heavy gravity planets and so they develop electronics. The reason you say invader force at all is because at some time along the line fairly early in its youth it took off to conquer the whole mest universe. You could expect almost anything in terms of physical form particularly physical form which matched the peculiar purpose of this group. They've usually got some gimmick like Fac One. Control has been the main thing. The way to control territory is to control people.

     2. there are five invader forces active and one aborning, but the one aborning is not active. It will probably be several million years before you see this one, some of you hit the track 60 trillion years ago mest universe and some of you didn't get into the mest universe until about 3 trillion years ago that is invader force one and invader force two [punctuation sic]. Now we don't see anything of invader force three here on earth. I just haven't found any threes. Invader force four is really holding the fort someplace or other. Every little while, a few million years, some planet will get taken over by an invader force.
[Dianetics & Scientology Technical Dictionary, 1975]

In 1957, the 5th London Advanced Clearing Course was held under the supervision of Hubbard and his son, Nibs. High on the agenda was the issue of past lives. A "scientific survey" was carried out, in which approximately seventy students investigated each other's past lives. Needless to say, all of them "found" that they had indeed lived many times previously (though one wonders what would have happened if a student had not "confirmed" Hubbard's theories). According to one of the students, Cyril Vosper, "when a student was having a lot of difficulty in making his story, or rather Past Life gel, Nibs would helpfully fill in bits." This perhaps accounts for some of the bizarre stories told in the 1958 book, Have You Lived Before This Life? , which after A History Of Man has to be one of the strangest books published this century.

If nothing else, the stories related by the book's contributors show that they had imaginations on a par with Hubbard's. Many recalled past lives on Earth (ranging from 19th-century Chinese coolies to Roman soldiers). Several remembered extremely strange extraterrestrial past lives. Ray Kemp wrote:

"This takes place nine galaxy periods ago. I was a male, born of space parents. I seem to have two or three mothers who died or were killed. At the age of five I was already on the look-out for brothels. At nine years of age I asked my father if I could join the space academy. However; this does not occur until I am 14. I am 15 when I go with other boys and girls for three months to learn all about sex and homosexuality. When I am 16 I kill my father while fighting on the planet and I join a space-ship. It seems I have a journey  here and rejoin the ship when I am 19. Then I learn all about space ship drill, take-offs, etc. There is homosexuality, as only officers are allowed women.

I did not care for homosexuality and soon gained the title of captain and so was able to have a wife of my own. She had a baby and a few days later I found the wife enjoying  pleasures with another officer. I put her and the officer up for trial and they were condemned and burned (zapped with special ray equipment). I killed the baby because I thought it was not my child. I wanted to go back home so I went to see the captain who was in charge of all the space-craft men and who knew where the ship was going. I asked for the space-ship to be turned around and he said "No." I went mad and killed the captain with my hands and broke up his body. Next I went into the main hall and pressed a button to ring the bell for assembly. I asked for votes for turning back the space-ship for home. Sixty-five per cent. said "Yes." As I was talking to the crew members I felt a gun at the back of my body and I was led off by officers along the corridor. I was screaming and struggling as I did not want to go to the Zap machine (a ray gun to destroy bodies).

However, I arrived and my body was held against the wall by clamps, hands were outstretched against the wall. This wall was made of special ray detecting material about a yard to two yards thick. I felt the warmth of the ray until it grew so bad that I left the body. As soon as the head had been burned off, the clamps were automatically opened and the body fell in a trench in the floor, arms outstretched. A large trap door made of metal was slammed on my arms cutting them off. The arms were swept into the trench and the trap door was lifted up again. As it slammed tight again, my body fell into a space container and was thrown outside by    tremendous pressure. A space coffin had its own power to fall away from the ship."

[L. Ron Hubbard (ed.), Have You Lived Before This Life?, 1958]

It's easy to see why this was such a traumatic event. Another Scientologist, Harry Dorfman, recalled something even more bizarre:

"There was a fantastic space factory with gold animals hanging concentrically from it all around, mainly elephants and zebra, by the necks. These appeared solid but periodically imploded or exploded. There was no gravity even near planets. Inside were four great bronze grinding wheels. During the incident, I look at time both ways as well as seeing it as a kind of circle from outside time. Therefore, it is hard to say whether discs from a case were ground up and made into small animals (which I think was the case) or whether animals were compressed into discs. I think the animals were subsequently inflated after blowing up through a totem and a cat devil and then broadcast (via the outside animals?) to other planets ...

The main and most awful part, and the most impenetrable, of the incident was the feeling of waiting and counting thirty to press a button. What was to happen then was uncertain. Either I was to blow up a planet, had blown one up, or failed to prevent it from being blown up. For this I felt I was punished by a bearded priest to whom I was betrayed by colleagues, by being compelled to work the grinder. There was also, towards the end, the stronger idea that all this appeared to happen in a robot body."

[L. Ron Hubbard (ed.), Have You Lived Before This Life?, 1958]

From the surreal to the sublimely ridiculous; another Scientologist recalled how he had been zapped by a Martian bishop:

The preclear was on Mars without a body 469,476,600 years ago, creating havoc, destroying a bridge and buildings. The people were called by an alarm to temple. PC [preclear] went and broke the back pew, and the Temple tower. He wandered in the town and saw a doll in a window, and got entrapped trying to move its limbs. People seized it, beat it up, and threw the doll out of the window (30 ft. drop). The doll was taken roughly to the Temple, and was zapped by a bishop's gun while the congregation chanted "God is Love." When the people left, the doll, out of control, staggered out and was run over by a large car and a steamroller. It was then taken back to the Bishop, who ordered it to be taken (in a lorry with others) to dig trenches or ditches for 2,000 years. (The whole incident took nearly 2,000,000 years.) Then it was taken and the body was removed and the PC was promised a robot body. The thetan (PC) went to an implant station and was put into an ice-cube and went by flying saucer and was dropped at Planet ZX 432. It was drawn to a building to an emanator. PC was interiorised by spinning and confusion into a dummy training and indoctrination robot body. In some way not very clear, a transfer was made to another robot body and PC was told to look after it for ever. It  reported to a village (after a doubtful encounter with a giant, and heat stroke) and was set to supervise unloading of saucers. It zapped and killed another robot and PC took over its body to prove it could work. The PC was punished in first robot in a saucer and shipped off. The saucer exploded en route and body of robot was in space falling in two parts with PC vainly endeavouring to take care of it and the second body. This was sucked by departure of a saucer into water in a dock. Divers brought it up, but the PC left it, he thinks, to attend the other body.

[L. Ron Hubbard (ed.), Have You Lived Before This Life?, 1958]

Other Scientologists recalled falling out of spaceships, being hit by meteorites, leading happy lives as walruses and even "falling in love with a robot decked out as a beautiful red-haired girl".

The book was originally intended to have a sequel, entitled Where Are You Buried? , in which Scientologists were to be invited to pinpoint the locations of their former bodies. But this project was abandoned -perhaps those consulted were unable to remember the details accurately enough. This was rather a pity, as it would have been well worth reading if it had matched the high (?) standards of Have You Lived Before This Life .

From http://wpxx02.toxi.uni-wuerzburg.de/%7Ecowen/essays/marcabs.html