Xenu rules! Hi, my name is Xenu and I'm an evil alien warlord bent on the destruction of Teegeeack (Earth, to you morons).
Here's my story:
Once upon a time (75 million years ago to be more precise) there was an alien galactic ruler named Xenu. Xenu was in charge of all the planets in this part of the galaxy including our own planet Earth, except in those days it was called Teegeeack.
Now Xenu had a problem. All of the 76 planets he controlled were overpopulated. Each planet had on average 178 billion people. He wanted to get rid of all the overpopulation so he had a plan.
Xenu took over complete control with the help of renegades to defeat the good people and the Loyal Officers. Then with the help of psychiatrists he called in billions of people for income tax inspections where they were instead given injections of alcohol and glycol mixed to paralyse them. Then they were put into space planes that looked exactly like DC8s (except they had rocket motors instead of propellers).
These DC8 space planes then flew to planet Earth where the paralysed people were stacked around the bases of volcanoes in their hundreds of billions. When they had finished stacking them around then H-bombs were lowered into the volcanoes. Xenu then detonated all the H-bombs at the same time and everyone was killed.
The story doesn't end there though. Since everyone has a soul (called a "thetan" in this story) then you have to trick souls into not coming back again. So while the hundreds of billions of souls were being blown around by the nuclear winds he had special electronic traps that caught all the souls in electronic beams (the electronic beams were sticky like fly-paper).
After he had captured all these souls he had them packed into boxes and taken to a few huge cinemas. There all the souls had to spend days watching special 3D motion pictures that told them what life should be like and many confusing things. In this film they were shown false pictures and told they were God, The Devil and Christ. In the story this process is called "implanting".
When the films ended and the souls left the cinema these souls started to stick together because since they had all seen the same film they thought they were the same people. They clustered in groups of a few thousand. Now because there were only a few living bodies left they stayed as clusters and inhabited these bodies.
As for Xenu, the Loyal Officers finally overthrew him and they locked him away in a mountain on one of the planets. He is kept in by a force-field powered by an eternal battery and Xenu is still alive today.