It is 1976. Seinfeld and Elaine and George are at high-school together. It’s a bit like Grease except none of the protagonists is young. They can be made to look young or they can look out of place among their peers.
The villain is Lex Toothsome played by Larry David. He wants to stop Seinfeld the Show, the one billion dollar show, ever being made. So he plays havoc with Jerry and Elaine’s love for each other and messes George about. Toothsome has a time-machine.
Luckily or otherwise, given his clumsiness, Cosmo Kramer is on to Lex Toothsome and his evil machinations.