Notified that he is twenty years late in returning a library book, Jerry insists that he brought it back, claiming to remember every detail about that very day. At the library with Jerry as he pleads his case, Kramer courts a librarian named Marion, and George claims that a homeless man on the library steps is Mr. Heyman, the gym teacher who he had fired from his high school. Meanwhile, after getting some strange reactions from her co-workers, Elaine is worried that she may be losing her job.

Returning to the Jerry's apartment, George recalls the incident in high school that led to Heyman losing his job. Warned by the library's investigator that he has seven days to return the overdue book, Jerry sets out to convince Lt. Bookman of his innocence. Jerry then tracks down the high school girlfriend he was with on the day he returned the book but when her recollections are totally different than his, he finally remembers giving the overdue book to George.

Marion's job is jeopardized when she and Kramer are discovered by Bookman during a late night visit to the library, while Elaine is sure that an upcoming appointment with her boss means that she is getting fired. After Jerry gives Bookman a check to cover the loss of the book, an embarrassing incident on the steps of the library convinces George that the homeless man is really Heyman. Finally, unknown to George and Jerry, Heyman has been holding the overdue book since losing his job twenty years ago.