"Moe Letter Blues" is the twenty-first episode of Season 21.
Homer, Apu, and Reverend Lovejoy take a day trip with their kids on Mother's Day so their wives can have a day to themselves, but the three of them receive a message from Moe saying that he is leaving Springfield...with one of their wives.
Moe's Tavern is the scene of merrymaking for the people of Springfield, and Moe relates how his role as bar tender gives him insight into his customer's lives. He notices tension in the marriages of Homer, Apu and Reverend Lovejoy. Mother's Day is approaching, and Marge suggests that Homer take the children on a cruise to Weasel Island. At first Homer is enthusiastic, but becomes concerned when Marge mentions his leaving will allow her to "take care of something." As the cruise ship pulls away from the dock, the three men receive a letter from Moe (via the Squeaky-Voiced teen mailman) informing them that he is running off with one of their wives.
At Weasel Island, the children spend time at a shoddy amusement park while the husbands agonize over their situation. At first, each insists their marriage is fine. Homer, however, remembers his mother-in-law's 80th birthday, where Moe was the bartender. Homer, who was displeased at Marge for only serving non-alcoholic beer, got into an argument with Patty and Selma and Marge told him that he ruins every event that she plans. Apu notices cracks in his marriage too, recollecting an incident where he and Manjula forgot their son Gheet at Moe's after using the bar's bathroom to change out of rain-soaked cricket uniforms. Manjula drove back to retrieve their son and did not return for hours. Homer mentions that he saw Manjula playing an interactive dance video game known as Zii Dance Dance Revolution with Moe that night. Finally, Reverend Lovejoy realizes that he ignored his wife Helen's needs, and Apu recounts how he witnessed Moe confiding in Helen that he was in love with a married woman and that Helen put her hand on his knee.
When the cruise ship returns to Springfield, each man realizes that they are equally likely to have lost their wives to Moe. Otto (who is completely stoned because every vehicle on the road looks like a monstrous character from Cars) drops Homer and his kids off first. At first, Homer thinks Marge is packing a suitcase but realizes that she has painted a portrait of her mother. Marge's mother forgives Homer for the 80th birthday incident, noting that there is "something evil" about Patty and Selma, and admitted that they even smoked during her pregnancy. Otto announces that Rev. Lovejoy is the next stop, and silences Kirk when he announces that he and Milhouse are the next stop. He reminds Kirk that there's nothing suspenseful at his place, admitting that even he had an affair with Luann. When Rev. Lovejoy arrives home, he too thinks Helen is leaving, but instead she surprises him with tickets to Istanbul by way of the Orient Express. By process of elimination, Apu concludes that his wife has left him. He arrives home to find Moe sitting with Manjula, but she tells Apu that Moe convinced her to salvage their marriage. In the end, Moe reveals that he saw how troubling the relationships were, so he organized Marge's portrait, the Lovejoys's trip and the Nahasapeemapetilons's marriage rescue. He explained that he wrote the horrible letter because the three men were taking their wives for granted, saying that they need to value them more just like their other family members. Moe admitted that he wished that he had someone who love him like the three men, instead of a one fling and done. Upon learning this, Homer admits that he does not know whether to punch or kiss Moe, but eventually chooses to do both as a thanks for teaching him that lesson. In the end, Moe then implores viewers to value their wives and mothers.
Homer says it's the first of the month, which means it cannot be Mother's Day.