Rich Man, Poor Man (1976)

S14E17 Take a Letter

To finance Stewie's private education, Lois takes a job as a postal worker. While sorting through the mail, she stumbles upon an unsent letter from Peter to his former girlfriend, Gretchen, which was written just a week before their wedding. When Gretchen attempts to create tension between Lois and Peter, Lois devises her own strategy to deal with the situation.