Marge Gamer is the seventeenth episode of Season 18.
Marge discovers the Internet and gets hooked on an online fantasy role-playing game called Earthland Realms, where she interacts with practically everyone in Springfield, including Bart, whose online persona is the local villain, the Shadow Knight. Meanwhile, Homer becomes a soccer referee, which strains his relationship with the one Lisa has when he has to call penalties on her.
Marge is ridiculed at a Parent-Teacher Association meeting for not having an e-mail address, and she chooses to purchase her own computer. She is taken by the Internet, and starts playing a role-playing fantasy game called "Earthland Realms", a parody of World of Warcraft, after she refreshes her e-mail and finds an ad for it. The game has her interacting with practically the whole town of Springfield, including Apu the gem trader, Skinner the turkey, Moe who looks like a troll but is not, Mrs. Krabappel the enchantress, Snake the Snake King, Chief Wiggum the pig man, Smithers the Barbarian, Comic Book Guy the fully armored crusader-like warrior, and Sideshow Mel as a creature who looks surprisingly like a Tauren. Suddenly, everyone hides, closes the windows of their houses and a knight in black atop a black horse. Apu grabs Marge and Skinner is flamed by the knight's flame sword. Apu tells Marge that the Knight in Black is the Shadow Knight, the most powerful and deadly character in the game. While off the game, Marge does the laundry and unintentionally hears Bart say "Another senseless killing by the Shadow Knight", learning that he is the Shadow Knight.
Meanwhile, Homer referees Lisa's soccer game and his subpar skills frustrate Lisa. Then, stung by her criticism, he learns the rules of soccer and becomes a better referee, impressing Lisa. However, Homer gives her a yellow card after Ronaldo points out that she is a "flopper", then a red card for bad sportsmanship when she rips the yellow card, causing her to get withdrawn from the game.
Marge goes to the Shadow Knight's castle and meets Milhouse the male cursed to look like a female servant, who got turned by a spell. In the online game, Marge constantly frets about Bart, who goes around causing carnage, and redecorates his "trophy room" with a Hello Kitty expansion pack. Bart smashes all of these decorations with his weapon and accidentally kills Marge's character in his fit of rage. After Marge and Lisa being mad at them, Homer and Bart go to Moe's Tavern to escape their troubles. Moe gives quality advice to Homer and Bart, prompting them to jokingly ask where the real Moe is. It is revealed that the actual Moe is tied up behind the bar. Acting on the fake Moe's advice, Bart makes up for killing his mother's character by reviving her with two-thirds of his character's health. While Elf Marge tends to the Shadow Knight, the other characters raid the castle to take advantage of his weakness and graphically kill him: they dismember and disembowel the Shadow Knight, then celebrate by propping his body upright and dancing around it like a Maypole, using his intestines for ribbons as they dance around it.
Homer gives Lisa a BBC documentary about "floppers" and she realizes that she was at fault. Marge refuses to come outside as the rest of the family plays football with Ronaldo. Feeling sorry for Bart, she becomes the new Shadow Knight and avenges Bart by eliminating the locals who participated in his elimination, first starting with Moe's character, while bouncing his head like a soccer ball.