The Simpsons' twenty-first season aired on Fox from September 27, 2009 to May 23, 2010. It was the first of two seasons that the show was renewed for by Fox, and also the first season of the show to air entirely in high definition.
With this season, The Simpsons established itself as the longest-running American primetime television series surpassing Gunsmoke.
The season received mainly positive reviews from critics, with many praising "The Squirt and the Whale", "To Surveil with Love" and "The Bob Next Door". The show moved up 16 positions in the Nielsen ratings from the previous season and received numerous award nominations, winning two - an Emmy Award for Anne Hathaway for her voicing in "Once Upon a Time in Springfield", and an Annie Award for "Treehouse of Horror XX".
Homer gets the lead in a movie about a superhero who can take on the powers of every superhero in any comic book he touches. To whip Homer into superhero shape, the studio hires a celebrity fitness trainer.
Mrs Krabappel is fired from Springfield Elementary after Bart spikes her coffee, causing her to make a drunken fool of herself. She is replaced by a cool new teacher, Mr. Vaughn, who is a quick hit with the kids. Bart, however, struggles with the guilt and considers telling Principal Skinner the truth.
Mixed martial arts (MMA) gains popularity in Springfield, and Marge leads a protest against it. The promoter of a local MMA company decides to gain publicity from the controversy by challenging Marge to a match, with the promise that he will shut down if she wins.
Lisa becomes livid with Miss Hoover, and in a series of homages to classic Hitchcock sets out for deadly revenge. Next, Krusty's latest fast-food sandwich transforms news anchor Kent Brockman into an enraged zombie and twenty-eight days later, cannibal zombies have overrun Springfield. In the final frightening fable, Homer accidentally falls through a trapdoor in Moe's tavern and impales himself on the pipes of Moe's microbrewery. Moe serves the barflies a glass of delicious beer flavoured with Homer's blood and Homer, now half-man, half-brewing apparatus, returns for vengeance.
Marge and a group called the "Charity Chicks" pose for a calendar in hopes of raising money for charity, but Marge becomes the talk of the town thanks to her racy poses. Meanwhile, Carl is chosen as the newest supervisor at the nuclear power plant, and hires Homer to be his personal assistant.
Principal Skinner is getting fed up with Bart's pranks, so he informs Bart that he is not the best prankster after all. A former student named Andy Hamilton is hailed as the best prankster, and Bart sets out on a mission to track Andy down. However, when Bart finds out that Andy is a 19-year-old still in his pranking days, they become fast friends.
After getting lost in a game of hide-and-seek, Lisa wanders into the field and discovers three teenage girls who are practising Wiccans. Though she is sceptical of their spells, the girls assure Lisa that they never hurt anyone and ask her to join their coven. Just before Lisa is inducted into their pact, Chief Wiggum arrives and arrests the three girls for witchcraft and Lisa becomes the star witness in the trial.
One snowy day in Springfield, Lisa informs Bart that she and Maggie share a bond that Bart will never understand because he doesn't have a brother, so Bart asks Homer for a baby brother. When Homer denies Bart's request, Bart makes his way to the Springfield Orphanage to find what he thinks he's missing.
Grampa is sitting on a bench waiting for his family when a thoughtful-looking man named Marshall Goldman approaches him and asks to hear about his life. Grampa tells Goldman about the time he was on a World War II battleship, which was hit by an enemy torpedo, and Goldman, a human-interest columnist, publishes it in The Springfield Shopper. Later, Grampa meets Mitch Albom and shares another story with Goldman for a follow-up article. Homer becomes jealous of Grampa's newfound fame and seeks a surrogate father. But when Homer discovers a draft of Goldman's third story, he must race to find Grampa before it is published and his life is changed forever.
Krusty is approached by two network executives who want to bring on female co-star Princess Penelope to increase the show's female demographic. The onstage and behind-the-scenes rapport between Krusty and Princess Penelope grows, and before long, Krusty asks for his co-star's hand in marriage. Meanwhile, when Mr. Burns puts a stop to the free doughnuts at the plant to cut costs, Homer, Lenny and Carl decide to meet with a headhunter who specializes in nuclear workers and opens their eyes to opportunities free of draconian doughnut-cutting measures.
Homer blows off Marge to buy a lottery ticket and winds up winning the million-dollar jackpot. Fearing how Marge will react if she finds out the reason Homer missed their date, Homer keeps his newly inherited fortune a secret and spoils his family with “anonymous” gifts. When Bart finds out, he and Homer spend the money like it’s going out of style, including buying front-row to tickets to see Coldplay in concert.
Homer takes Marge out for a romantic evening of ice skating and hand-holding, but upon entering the rink, they encounter a curling team practising. Marge and Homer take to the ice and discover their love for the sport, and soon after, join the curling team and compete with them in the Olympic trials. Team Springfield claims the win and moves on to the 2010 Vancouver Games where Bob Costas covers the action. Meanwhile, sleazy vendors introduce Lisa to the world of collecting Olympic pins, and before long, Lisa is hopelessly addicted.
When Miss Hoover asks her students to research their family history, Lisa is horrified to discover that most of her ancestors were bad people – a motley crew of horse thieves and deadbeats. But while rummaging through the attic, Lisa happens upon a diary kept by her ancestor, Eliza Simpson. As Eliza’s story unfolds, Lisa learns that her family was part of the Underground Railroad, a group that helped slaves escape to freedom. Eliza recounts liberating a slave named Virgil, but when Lisa presents her findings at school, some of her classmates refute it, leaving Lisa determined to exonerate her family’s name.
When Bart fails to turn in his homework, Mrs. Krabappel sends a letter home about Bart's behaviour. Despite Bart's best efforts to intercept it, Homer reads the letter, and he and Marge visit Principal Skinner for a parent-teacher conference. Furious, Homer punishes him, but Marge takes a more sympathetic approach. When Bart realizes he can pit Homer and Marge against each other to his benefit, his scheming reaches new heights.
Principal Skinner announces that Mrs. Krabappel was called out of town and budget cuts dictate that, until she returns, the school's two fourth grade classes will merge. Bart reluctantly shares a desk with Nikki and develops a flirtatious rapport. Bart talks to Grampa about his new crush, and at Grampa's suggestion, gives Nikki a kiss. But when Nikki starts sending Bart mixed signals, he swears off women forever. Meanwhile, Lisa's classmates ostracize her for being an overachiever, and First Lady Michelle Obama, a self-professed nerd, comes to Lisa's defence.
When Homer is playing noisily in the yard, it disrupts Flanders' bible-study group. Coaxed by the reverend, a frustrated Flanders takes it upon himself to redeem Homer by inviting the Simpson family on his church retreat to Jerusalem. Unappreciative of the history and culture, Homer would rather hang out at the hotel's breakfast buffet rather than tour the city. But when an eccentric tour guide takes the group to famous monuments, including the Dome of the Rock and the Wailing Wall, Homer finally proves he is not beyond salvation.
When the police are called to diffuse a rowdy crowd at Mr. Burns' estate, one of the officers recognizes priceless stolen paintings on the walls, and the maniacal billionaire is taken downtown for questioning. With Mr. Burns gone, Smithers takes charge of the power plant. But when employees take advantage of his good nature, he exacts revenge by forcing the employees to work night and day. These unfair working conditions prompt Homer and his crew to devise a plan to bust out Mr. Burns.
Homer is completing his court-ordered community service when he befriends his supervisor, Chief Wiggum, by offering him one of his sandwiches. Touched by the act of kindness, Wiggum assigns the other convicts unpleasant tasks, but allows Homer to join him at the picnic table. They continue to grow close, but when the Chief gets injured during a botched bank robbery, Homer doesn't come through when Wiggum needs him the most. Meanwhile, Bart becomes addicted to Battle Ball, a Japanese game made up of plastic balls and magnetic cards, and his family and teachers try to help him kick the habit.
The Simpsons decide to embrace a cheaper, alternate source of energy by erecting a wind turbine in their backyard. But when Homer realizes some of the power is being directed to the local electric company, he decides to remove his home from the grid and becomes completely dependent upon an unreliable source of power. Meanwhile, a storm erupts, trapping a magnificent 150-foot-long blue whale ashore, and Lisa and Homer attempt to help the poor creature.
A bomb squad mistakenly blows up Homer's unattended gym bag, releasing radiation into the city and authorities react by suspending civil liberties. Wiggum and his men install surveillance cameras around Springfield and round up suspected terrorists, including groundskeeper Willie, but when monitoring the nonstop flow of video imagery proves to be too much, Wiggum enlists concerned citizens to help keep the city safe. Meanwhile, Lisa becomes fed up with being blonde, so she dyes her hair a dark color.
As Mother's Day approaches, Moe narrates an episode in which he writes a letter to Homer, Apu and Reverend Lovejoy, who are vacationing with their children, and threatens to run away with one of their wives. While the trio tries to determine whose wife Moe is referring to, Homer, Lovejoy and Apu flashback to the intimate moments they initially ignored between Moe and Marge, Manjula and Helen Lovejoy. But when the boys return from their trip, they're in for the surprise of their lives.
Bart becomes convinced that his new neighbor, Walt, is his archenemy, Sideshow Bob, disguised and back for revenge. But when Marge tries to convince Bart otherwise by taking him to visit the state penitentiary, a disturbing truth is revealed.
When Moe discovers a hidden talent for judging contests, he is approached by a television agent to join the AMERICAN IDOL judges' panel. Moe flies to Los Angeles where he tours the Fox lot and receives some sage career advice. Meanwhile, Homer drives Marge crazy when he begins hanging around the house.