Kristen Wiig ("Despicable Me 2") guest-voices in the season premiere episode as an intense, high-strung FBI agent whom Lisa enlists for help when she suspects that something sinister has happened to Homer.
Also, Elizabeth Moss ("Mad Men") plays a grateful mom who names her baby Homer Jr., after Homer Sr. helps in the delivery of the child.
The opener of the 24th annual "Treehouse of Horror" is from Guillermo del Toro and features monsters and classic characters from horror films throughout history.
In the first of three spine-tingling stories, Homer rides around Springfield wreaking havoc in a rhyming take on a popular children's tale.
In "Dead and Shoulders", Bart is beheaded during a kite accident, his head is attached to Lisa's body and they must live together as one.
The final terrifying tale, "Freaks no Geeks", features Mr. Burns' traveling circus, The Burnsum and Bailey Circus, which has stopped in Springfieldland in the 1930s. Trapeze artist Marge and Strong Man Homer are performers and things go awry when circus freak Moe starts to make advances towards Marge.
Four Springfielders are prompted to try to right past regrets following the funeral of a beloved neighbor. Homer regrets selling his Apple stock to buy a bowling ball, Marge worries she caused Bart's rebellious streak, Mr. Burns reminisces about a romance he gave up with a beautiful Parisian woman and Kent Brockman admits he was too afraid to leave local news and follow his former partner Rachel Maddow to cable news.
Marge encourages Homer to embrace his "You-Only-Live-Once" spirit by inviting an old pen pal to stay with him and cross items off of his "to-do" list from when he was a 10-year old.
Meanwhile, Lisa institutes a new honor code at Springfield Elementary.
Homer remembers his Lamaze skills when he's forced to deliver a baby in an elevator. When the mother (guest voice Elizabeth Moss) names the child Homer Jr., he quickly develops a deep emotional bond with the kid.
Meanwhile, Lisa is recruited by the cheerleaders of the local pro football team, the Springfield Atoms.
Lisa finally makes a new best friend, Isabel Gutierrez, only to discover that she's a Republican and her opponent in the campaign for second grade class representative
Principal Skinner promises the students that the best-behaved among them can take a ride in a submarine. When Skinner gives all the kids a clean slate, Bart believes even his own past indiscretions will be forgotten if he doesn't get into any more trouble.
Meanwhile Lisa tries to help a cash-poor Krusty to turn things around by suggesting he sell the foreign rights to his shows
Springfield is inundated with holiday revelers after the nuclear power plant springs a radiation leak, causing snow to fall all over town. When the tourists drive up Christmas shopping prices, Marge opens up the Simpsons' house to boarders.
Homer becomes annoyed with movie theaters, so Bart teaches him how to download movies illegally. Homer is delighted with his discovery of free movies until he's caught for piracy.
Comic Book Guy realizes his comics are nothing if he can't share them with anyone. Then he meets Kumiko, a Japanese woman writing an autobiographical manga, and he seeks out Homer's advice on how to date her, ending happily for everyone.
Mr. Burns gives high-tech eyeglasses to all of his employees in order to spy on them. Homer loves the enhanced reality of his new gadget - that is, until Marge tries them on and he finds out she's seeing a marriage counselor. Meanwhile, Bart takes a stand against buying Nelson a Valentine's Day card, despite buying one for every other kid in class, and Nelson delivers him an ultimatum: find him the best Valentine's Day gift ever or be fed to the classroom electric pencil sharpener.
On a research trip, Lisa is shocked to discover that Sideshow Bob is now the chief scientist at a massive chemical engineering company, but her fears are allayed as she becomes enamored with his likeminded appreciation of high-culture.
Meanwhile, Marge's attempt to preach healthy sexual practices to a teen church group goes awry.
Bart is rescued from a confrontation with Springfield Elementary's bullies by Diggs, a recent transfer student and aspiring champion in the niche sport of falconry. But, Bart soon discovers that Diggs wants to do more than just wrangle falcons: he wants to take to the sky himself.
Marge invites Grampa and two other octogenarians to live at the Simpsons' house after the Retirement Castle is closed for health violations, but she quickly becomes frustrated with Homer when he starts embracing the "old person lifestyle".
Meanwhile, at Springfield Elementary, Bart defends Nelson's affinity for hand-me-down underwear, leading Nelson to befriend Bart
After Lisa's new pet guinea pig destroys the Simpsons' living room art, Marge falls in love with a beautiful painting at the Van Houten's yard sale, which Homer snaps up for 20 bucks.
But, when Lisa reveals it's by a famous early 20th century painter and could be worth 0,000, Marge and Homer face a dilemma: split the money with the Van Houtens, or keep the cash as a "cushion" for their kids.
Homer is recruited as a World Cup referee after Lisa extols his integrity on the youth soccer field in a Springfield Elementary speech contest.
But when the Simpsons travel to Brazil, Homer finds his honesty tested by one of South America's slickest gangsters in the soccer match-bribing business.
Marge is concerned that Lisa is dating below her station when she brings home competitive eater-in-training Lucas Bortner, and she enlists Homer to help Lisa explore other options.
Meanwhile, Bart is thrilled to receive free gifts from Snake Jailbird after he helps the convict escape from a jam, but when a betrayal from Milhouse puts Snake back in jail, Bart must forge a plan to set him free.
Set in the future, Homer has a new clone for every time he dies. Lisa is married to a zombie version of Milhouse, while Bart deals with custody issues with his ex-wife, Jenda. Each defines what love means to them.
When Bart develops powers that help Springfield couples conceive, the mafia kidnaps Homer and Bart in hopes of birthing a thoroughbred race horse.
Homer awakens to find his family and everyone in Springfield are made of Legos, and finds he must discover how he got in the Lego world and how he can get back home before he is left in a world of bricks for all of eternity.
Marge swears off befriending any more couples when Homer offends their charming new British neighbors.
But when Lisa declares that she, too, doesn't need friends, Marge reconsiders.
Bart is plagued with guilt when he wins the annual "last day of school" race around Springfield Elementary, with an assist from Nelson, who beats up frontrunner Milhouse.
Meanwhile, Homer tries to bring back the annual 4th of July fireworks display after it's canceled for budget reasons.