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".
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.
Homer is cast as the lead in Everyman, a super hero movie written by Comic Book Guy. But the newly christened actor has trouble staying on the exercise and diet regimen that his provided fitness trainer lays out for him.
When Mrs. Krabappel confiscates student's cell phones, the class, organized by Bart, decides to teach her a lesson by spiking her coffee and watching her make a drunken fool of herself. Principal Skinner fires Mrs. Krabappel and replaces her with a cool new teacher, Zachary Vaughn, who impresses Bart and his friends with a hip attitude and love of texting, blogging and using Twitter and Facebook. Torn by his affection for Mr. Vaughn, yet plagued by guilt, Bart makes his way to Skinner's office to tell him the truth and get Mrs. Krabappel's job back.
Marge protests a violent sport.
In the 20th anniversary of THE SIMPSONS annual "Treehouse of Horror" Halloween special, three chilling tales of Halloween horror unwind. First, 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 flavored with Homer's blood and Homer, now half-man, half-brewing apparatus, returns for vengeance.
Marge shows some skin for charity.
Bart discovers there was an even more awesome prankster than he, and determines to track him down, only to find he is a sad loser living in his mother's house (and voiced by Jonah Hill).
Lisa joins a group of teenage wiccans, only to wind up in a witch trial conducted according to ancient Springfield laws.
Bart wishes for a brother this year.
Grampa meets a newspaper columnist who chronicles his life in the local paper. Meanwhile, Bart takes home a beloved class doll, and Lisa loses it down a sewer.
In the landmark 450th episode of THE SIMPSONS, two network executives bring on female co-star Princess Penelope (guest voice Anne Hathaway) to increase the Krusty’s female demographic. Krusty falls head over heels and asks for his co-star’s hand in marriage. Meanwhile, when Mr. Burns puts a stop to the free donuts at the plant and Homer, Lenny and Carl look for job opportunities free of draconian donut-cutting measures.
Homer (Dan Castellaneta) blows off Marge (Julie Kavner) 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 (Nancy Cartwright) finds out, he and Homer spend the money like it’s going out of style, including buying front-row to tickets to see Coldplay (guest voice Martin) in concert.
After discovering their love of curling Marge and Homer try out for the Olympic team and actually make it to the 2010 Vancouver Games. Meanwhile, in Vancouver, Lisa develops an addiction to collecting Olympic pins.
Lisa discovers the diary of an ancestor that reveals the Simpson family was in the underground railroad before the Civil War.
When Bart realizes he can exploit Marge and Homer's differences, it nearly breaks their marriage apart.
Bart wrestles with matters of the heart.
When Homer (Dan Castellaneta) disrupts Flanders’ (Harry Shearer) bible-study group, he 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 than tour the city. But when an eccentric tour guide (guest voice Cohen) takes the group to famous monuments, including the Dome of the Rock and the Wailing Wall, Homer finally proves he is not beyond salvation in the all-new “The Greatest Story Ever D’ohed” episode of THE SIMPSONS.
Sasha Baron Cohen (“Borat”) Makes Guest Voice Appearance.
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 (Dan Castellaneta) and his crew to devise a plan to bust out Mr. Burns.
Homer (Dan Castellaneta) is completing his court-ordered community service when he befriends his supervisor, Chief Wiggum (Hank Azaria), 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 (Nancy Cartwright) 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, alternative source of energy by erecting a wind turbine in their backyard. But when Homer (Dan Castellaneta) 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 (Yeardley Smith) and Homer attempt to help the poor creature.
When a bomb squad mistakenly blows up Homer’s (Dan Castellaneta) unattended gym bag, radiation is released into the city, and authorities react by suspending civil liberties. Wiggum (Hank Azaria) and his men install surveillance cameras around Springfield and round up suspected terrorists, including groundskeeper Willie (Dan Castellaneta). 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 (Yeardley Smith) becomes fed up with being blonde, so she colors her hair.
As Mother’s Day approaches, Moe (Hank Azaria) narrates an episode in which he writes a letter to Homer (Dan Castellaneta), Apu (Hank Azaria) and Reverend Lovejoy (Harry Shearer), 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 (Julie Kavner), Manjula (Jan Hooks and Tress MacNeille) and Helen Lovejoy (Maggie Roswell and Marcia Mitzman Gaven). But when the boys return from their trip, they’re in for the surprise of their lives.
Sideshow Bob returns for revenge.
Moe becomes a judge on American Idol.