The entire group stand trial under a "Good Samaritain" law that requires you to render assistance to people in the event of a hold up or robbery. They end up going to jail for failing to come to the assistance of a robbery victim, and the jury is horrified at the callous attitude of them all as they laugh and make jokes -- the whole thing being recorded on videotape unfortunately.
It's out of character for them to make fun of such a serious situation. It's a silly way for it all to end, and you'll always remember them as being locked up in jail. If you can at all help it -- skip this episode, and remember Seinfeld for the great sitcom it was.