Season 21 of The Simpsons kicks off with an episode co-written by Seth Rogen, who also guest stars. The rest of the season celebrates The Simpsons 20 years on TV.

"Homer the Whopper"
9/27/09
Comic Book Guy casts Homer as the lead in "Everyman," a feature film based on a new comic book superhero who possesses the ability to take on the powers of every superhero in every comic he touches. To whip Homer into superhero shape, the movie studio hires celebrity fitness trainer Lyle McCarthy (guest voice Rogen), but Homer has trouble sticking to his new healthy regimen.

"Bart Gets a 'Z'"
10/4/09
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.

"The Great Wife Hope"
10/11/09
The men and boys of Springfield become obsessed with Mixed Martial Arts, a violent sport where fighters inflict severe pain on one another. When Marge and her friends stage a protest, the show's promoter embraces the controversy and challenges Marge to a match, promising to shut down the competition if she wins.

"Treehouse of Horror XX"
10/18/09
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.

"The Devil Wears Nada"
11/15/09
In an effort to raise money for charity, Marge and the "Philanthro-Chicks" decide to pose for a calendar. At the photo studio, Marge loosens up with wine and before long, Marge and her pin-up poses are the talk of Springfield.

"Pranks and Greens"
11/22/09
Bart tracks down Andy Hamilton, Springfield Elementary's best prankster ever (guest voice Jonah Hill) and they become fast friends until Bart gets Andy a job working for the ultimate clown.

"Rednecks and Broomsticks"
11/29/09
After getting lost in a game of hide-and-seek, Lisa wanders into the field and discovers three teenage girls, including Cassandra (guest voice Neve Campbell), who are practicing Wiccans. Though she is skeptical 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. Meanwhile, Homer starts hanging out with Cleatus and his hillbilly friends when he discovers that they make moonshine.

"O Brother, Where Bart Thou?"
12/13/09
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. Guest voice are Peyton Manning as himself; Eli Manning as Himself; Cooper Manning as Himself; Kim Cattrall as Herself; Jordan Nagai as Charlie, Huell Howser as Himself; The Smothers Brothers as Themselves.

"Thursdays with Abie"
1/3/10
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 (guest-voicing as himself) 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.

"Once Upon A Time in Springfield"
1/10/10
In the landmark 450th episode, Krusty is approached by two network executives who want to bring on female co-star Princess Penelope (guest voice Anne Hathaway) 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 donuts 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 donut-cutting measures. Also guest stars Jackie Mason as Rabbi Krustofsky and Eartha Kitt as Herself.

"Million Dollar Maybe"
1/31/10
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 (guest voice Chris Martin) in concert.