Marge mistakenly receives breast implants, to her dismay (but to Homer's delight), and embarks on a new career as a trade show model. Meanwhile, a stunt by Bart results in a big public-image problem for Krusty, and Bart hatches a plan to help him. Bart's plan goes awry, but Marge's enhanced "assets" end up saving the day.
Lisa and Homer are watching a historical documentary and make a bet on who will win the 1948 U.S. Presidential election. Homer makes the mistake of believing the polls that predicted victory for Governor Dewey, so Lisa wins the bet and she gets to choose the activity for Daddy-Daughter Day. Her activity is taking part in building homes for Habitat for Humanity alongside former U.S. Presidents Jimmy Carter, Bill Clinton, and George H. W. Bush (who parody The Three Stooges). While painting, Homer attempts to create a similar alias to Bart's El Barto, but the idea backfires when a Latino man mistakes Homer for a homosexual due to the alias. Homer frantically paints over his graffiti tag, and takes off his wedding ring to keep from getting paint on it (he also admits that he had to make a crude imitation of a wedding ring from aluminum strips and Band-Aids because the real ring was eaten by a turtle). Lindsey Naegle and Cookie Kwan mistakenly think that Homer is recently divorced ("Hello! Wedding ring tan line! He's new on the market.") and begin flirting with Homer. Marge, driving by, sees Homer appearing to flirt with the women (but he is actually explaining how Marge gave birth to Bart). She worries that Homer has lost interest in her, so on Manjula's advice she decides to get liposuction.
Unfortunately, a mix-up results in Marge getting breast implants, which were intended for one of Mayor Quimby's interns. The doctor says that Marge must wait for 48 hours before the implants can be removed, and Marge is furious with the new implants.
Meanwhile, Bart and Milhouse watch an old episode of Batman, featuring Krusty as a villain named ClownFace, who threatens Batman and Robin with a rapidly spinning carousel of death. Bart and Milhouse like the idea and are inspired to recreate the stunt, with Bart putting Milhouse on a merry-go-round in school. Otto uses the bus to kick-start the merry-go-round, spinning it at an incredible speed. The bolts give way and the merry-go-round goes flying through the air, knocking over the school flag and a group of Iwo Jima veterans visiting the school. After the merry-go-round comes to rest, a dizzy Milhouse vomits on the flag, shocking the veterans. Principal Skinner catches Bart for it and demands an explanation, to which Bart is forced to admit that he and Milhouse saw Krusty do the stunt on TV. In response, Skinner leads a group protesting against Krusty, who is now seen as a dangerous influence to children. Krusty’s show is revamped to exclude all possibly dangerous elements, destroying the show's popularity with children, and making Lisa angry at Bart for ruining Krusty's reputation.
After coming home from her surgery, Marge realizes that her very large breasts are making her life difficult, as they get in the way when she tries to do things like open the cupboard and pick up Maggie to comfort her when she falls down. Marge tries to hide her breasts from her family, but Homer and the kids find out Marge's secret after a few short hours. Homer, Marge and the kids go out for dinner to Luigi's so Homer can flaunt the new Marge. Luigi Risotto, who has just turned away the Flanders family, immediately admits the Simpson family after seeing Marge’s breasts, and offers to make her a special pasta instead of the "crap from a can" he serves to everyone else. Clearly, Marge's new figure attracts male attention, and even the lobsters in Luigi's lobster tank seem to take notice of her. Homer soon starts singing about Marge’s breasts, and is joined in a musical number by several other male patrons (You're a whole bunch of stuff). Kiki, a trade show executive, approaches Marge and offers her a modeling job from A.C. Milan. Marge accepts and initially enjoys the experience, but later experiences back pain because of her heavy breasts. She also finds herself being ogled, pinched and leered at by every man in Springfield.
At the Springfield Shoe Expo, Marge is further humiliated by lustful men while promoting shoehorns. At the same time, Bart makes a plan to help Krusty to win back his popularity, using Milhouse and Stampy (Bart's old pet elephant). The plan is for Stampy to appear to threaten Milhouse while Bart calls for help. Then Krusty is to respond and use Stampy's safety word ("magumbo") to get Stampy to roll over on his back and leave Milhouse alone; then, Krusty can take the credit for rescuing Milhouse and win back his reputation. Unfortunately, things quickly get out of control when Stampy stuffs both Milhouse and Bart in his mouth. Homer tries to save them, but also ends up stuffed into Stampy's mouth. Also, Krusty forgets Stampy's safety word, making matters even worse.
The police decide to shoot at Stampy, which would endanger Homer, Bart and Milhouse. Marge appears on the scene, and after unsuccessfully trying to dissuade the police from shooting, distracts the police by revealing her huge breasts to the crowd. Krusty saves the day when he looks at Marge's breasts and accidentally says Stampy's safety word: "Wow! Look at those magumbos!" Stampy releases Bart, Milhouse and Homer, and Krusty is hailed as a hero and his good image is restored. The next day, The Springfield Shopper proclaims that Krusty saved the day, in a very small article that's barely a paragraph long, while a huge headline reads "Crazed mom goes topless!", promising 25 pages' worth of pictures inside.
Marge finally has the implants removed, and is greeted by a surprisingly happy Homer who proclaims that it's good to be able to get his arms around her to hug her again.
"Large Marge" received positive reviews from critics and fans who praised the episode for putting Marge in the spotlight and making her a funny character. The episode has a 7.2/10 user rating on IMDB. Despite this, most fans do consider this episode one of the worst ever made, due to the crude sex jokes and weak main story.