Here's a condensation of the basic theory from moderator/conspiracy advocate Alex Jones' PrisonPlanet.com:
The planes did not bring those towers down; bombs did. So why use planes? It seems they were a diversionary tactic - a grand spectacle. Who would want to divert our attention from the real cause of the collapse of those towers? It must be those who benefited most from these attacks. ... All evidence points to elements inside, high atop the governments of Israel and the United States. Those wishing to implement their world government through their control over finance, media and militaries are guilty of these most heinous crimes. This atrocity is proving to advance the domestic police state agendas and consolidate the Middle East’s oil reserves. The people who most benefited from these attacks are the wealthiest, most privileged and powerful men on earth who feel they will finally be able to hold dominion over the nations of the world through their New World Order. For some of these globalists, these attacks were merely a means to an end - a huge step toward ruling a world socialist system. But for others, likely the planners of such a vile crime, this was a mass sacrifice to themselves. This mass ritual sacrifice of the vulnerable and the heroes who tried to save them was perpetrated by power crazed freaks who are simply - satanic.The panel featured 9/11 Scholars for Truth founder James Fetzer, BYU Physics Professor Steven Jones, President of the Institute for Space and Security Studies Dr. Robert M. Bowman, USAF Lt. Col. (Ret.), filmmaker and radio talk-show host Alex Jones, and terrorism expert Webster Tarpley.
The crowd at the 9/11 conspiracy conference had their own set of facts: That the WTC could only have collapsed because explosives had been pre-positioned there; that news media is complicit somehow; that government officials were told in advance of 9/11 to stay off airplanes that day; that the 9/11 Commission's work was a whitewash sham; that the real airplanes were hidden, and exact replicas were used; that no airplane debris was ever found in Pennsylvania or at the Pentagon -- where they believe a cruise missile hit; and that the U.S. military had a shockingly -- read, deliberately -- slim defensive posture that day.
Let's get one thing straight: Even the federal government's version of 9/11 is a "conspiracy theory," but these folks have collected "facts" -- regardless of their accuracy or significance -- and congealed them into an ugly mass of hurtful tripe-for-profit.
Even if these wacko theories held any water, anybody who has dealt with the criminal justice system knows that the only perfect crime is one in which only the perpetrator knows what happened, leaves no evidence and keeps his mouth shut. If two people know, the chances of it remaining a secret go down. So how many people do you think would have to be involved in the 9/11 attacks? What would be the chances that all of them could keep their mouths shut? That none would experience a pang of guilt that would cause him to confess? There is no chance.
For a point-by-point refutation of the diverse WTC conspiracy theories floating around, check out this site.
Human nature leads to these conspiracy theories, and in this case, human nature proves them wrong.