THIS IS WHAT David S. Lifton asks you to believe: President Kennedy was killed by "plotters" -- some people in the executive branch, including members of the Secret Service -- who shot from the front. Then they altered the evidence, that is, they did surgery on the president's head and torso, to make it appear that all shots came from the rear, thus setting up Lee Harvey Oswald as the patsy. To carry out the alterations, the plotters stole Kennedy's body from its ornate coffin before Air Force One, returning from Dallas, landed at Andrews Air Force Base. While TV-watchers around the world saw the coffin -- now empty -- being unloaded from one door of the plane, the plotters were sneaking Kennedy out another door in a body bag. They loaded him aboard a helicopter for a quick hop to Walter Reed Army Medical Center, where his brain was removed and the necessary changes were made to it and to Kennedy's throat and back. After spending half an hour forging wounds, the plotters sent Kennedy over to Bethesda Naval Hospital, where an official autophsy was scheduled. Lifton isn't sure whether the brain had been replaced or whether it was shipped separately, under a sheet, disguised as a stillborn baby; he seems to favor the latter theory.

At Bethesda another elaborate shell game -- complete with decoy helicopters, a decoy ambulance, and a decoy coffin -- was carried out in order to get Kennedy back into the Dallas coffin before Mrs. Kennedy and the funeral party became aware of the switcheroo. Finally, an official autopsy was performed by Navy doctors who -- if I understand Lifton correctly; I frequently feel that I don't -- were so stupid they didn't notice either that Kennedy's brain was missing or that other doctors had recently been cuttin on him.

(Lifton has a back-up theory. If Kennedy's head wasn't tampered with at Walter Reed, then it was altered in a secret "pre-autopsy" autopsy at Bethesda, though he is not accusing those honest-but-dumb Navy doctors of having a hand in it.)

If nothing else, Lifton can claim to have developed probably the grisliest and most bizarre of the countless Kennedy assassination cover-up plots. But what really makes this book interesting is not so much his razzle-dazzle theories as the sheer fanatical energy that rolls through these 747 sometimes dense and tangled pages -- a torrent of interviews, rumors, personal musings, egotistical flights, reinterpretations, recollections of feuds with other assassination critics, innuendos, charts and history.

But readers who aren't drowned by all that will begin to notice that Lifton isn't always quite fair. Just like the Warren Commission, he doesn't pay much attention to good witnesses who disagree with the outcome he has decided on.

For example, there were two lab technologists -- James Jenkins and Paul O'Connor -- who helped prepare Kennedy's body for the autopsy and assisted the surgeons during the investigation. Years after the event, Lifton interviewed both. He says Jenkins "insisted" Kennedy's brain was there and that he himself had performed the usual autopsy treatment of it. O'Connor, on the other hand, gave Lifton loads of such statements as: "he didn't have any brains left"; "there wasn't anything to remove"; "the cranium was empty"; "there was no brain on the body, near the body, or in the casket, or anything that I know of."

Lifton decided to go with O'Connor and make him the foundation for the purloined-brain theory. Jenkins is brushed off and the reader gets to hear virtually nothing from him, although Lifton concedes that Jenkins, who had several years of advanced medical training after leaving the navy, was "different from the other personnel I hunted down on this project" because "he was a keen observer." So why pick O'Connor over Jenkins? That kind of selectivity strikes me as coming very close to cheating.

After Lifton's wild tour of the military morgues, The Plot to Kill the President (to be published in April) comes across with all the excitement fo a wheat market report. The Plot grew out of the costly and often clumsy investigation of the House Select Committee on Assassinations, which G. Robert Blakey served as chief counsel and staff director and Richard N. Billings (a Life magazine alumnus) as editor. Anyone who kept up with the newspaper accounts of their committee's work will already know broadly the conclusions of The Plot -- President Kennedy was done in by the mob, which coordinated its assassins, firing from the nearby grassy knoll, with Lee Harvey Oswald, firing from book depository. The most impressive work done by the House committee was to develop acoustical evidence that shots were fired from more than one direction. Jack Ruby is presented -- elaborately -- as the mob's hit man, to silence Oswald.

Motive? Well, having helped rig the Chicago ballots that put Kennedy in the White House, the mob felt betrayed when he named his brother Bobby to head the Justice Department and then Bobby began a heavyweight crackdown on organized crime. But what really made Kennedy vulnerable, say Blakey and Billings, was that he stooped to the mob's level in his sex life, even to sharing a girlfriend with Mafia kingpin Sam Giancana -- a liaison that B&B see as "Kennedy's fatal flaw, the error in judgment for which the gods would demand their due . . . From the mob's point of view, Kennedy had been compromised. He had crossed the line."

Actually, the stuff I like best in The Plot is incidental, such as the story about how the Secret Service, fearing a Dallas ambush of Lyndon Johnson, used Mrs. Johnson as a lure to draw potential assassins' fire. You've come a long way, baby.