DISCLAIMER: If you do NOT want to know what happens on the last show, should you prefer to be surprised then read no further.
OK. You've been warned.
Here it is, the basic outline.
Harkening back to the premise of Jerry trying, from the beginning, to land a sitcom, the last episode begins with the news that his "show about nothing" has been picked up.
Jerry, George, Elaine, and Kramer hopt a plane for Los Angeles to further this move towards actually making the show. Shortly after leaving, however, the plane they are on must make an unanticipated stop in Massachusettes (it's Kramer's fault, that is all I know as to WHY this happened). While in a town in Massachusettes, the four of them happen to encounter a car-jacking. While one of them films the car-jacking the others make wry comments about the nature of a car-jacking.
Others see this happening and when the police arrive on the scene the four are charged under the new Princess Diana law that says you cannot take photos/videotape at the scene of an accident/crime when there there are lives in danger.
Charged with this crime, the four of them get a jury trial. This trial is the premise used by the show's writers to reintroduce/have flashbacks with some of the major guest-stars (i.e Terri Hatcher, Keith Hernandez). These people appear during the trial as character witnesses *against* the cast!
The secretly-taped final endings revolve around whether the group is acquitted, charged, hung jury, whatever. Well, they end up being guilty--not just of the crime but of generally belittling and harming the lives of so many guest stars.
There it is. Don't ask how I learned this. Birds whisper. The above is fact. Hope you enjoyed it!