Morticia Addams calmly gives birth to a baby boy, Pubert. When Wednesday and Pugsley play their ooky games with him, but he narrowly avoids death every time. Gomez and Morticia try to hire a nanny, but the children frighten all the prospects away. The last applicant, Debbie Jellinsky, seems to be made of sterner stuff. Unbeknownst to the Addams, she is also the serial killer known as "The Black Widow". America's Most Disgusting Unsolved Crimes reveals that Debbie has been traveling in disguise for quite some time, marrying rich men and killing them for their fortunes. Now she has set her sights on Uncle Fester and the vast Addams fortune. When Wednesday becomes suspicious of Debbie's interest in Uncle Fester's money, Debbie tells Gomez and Morticia that Wednesday and Pugsley want to go to Camp Chippewa. The parents are horrified at the thought, but agree to send them.
With the kids out of the way, Debbie marries Uncle Fester, then promptly tries to kill him on their Hawaiian honeymoon by electrocuting him in the bathtub. However, our electrified Uncle Fester survives and mistakes her murderous actions for ordinary affection. Debbie denies him sex until he promises never to see his family again. In anguish, he agrees. The couple moves into a garish mansion in the suburbs.
With Uncle Fester gone, Gomez goes into a depression and Pubert becomes "possessed", turning blonde, rosy and cheerful. Meanwhile, at Camp Chippewa, Wednesday and Pugsley do not fit in with the rest of the wealthy, snobby campers. During lifesaving training, Wednesday intentionally lets her partner, Amanda Buckman, almost drown in the lake. At night, during the telling of ghost stories, Wednesday tells of a ghost undoing all of the campers' nose jobs overnight, which causes her fellow campers to scream in terror while she looks on, smirking with satisfaction. The Addams children try to escape at night when they receive word of Uncle Fester's marriage to Debbie, but are caught. Amanda and the rest of the campers suggest that the Addams kids be punished, but the camp counselors Gary and Becky, would prefer to inspire them by singing "Kumbaya", which makes Wednesday and Pugsley cringe. Meanwhile, Wednesday encounters a soulmate of sorts in Joel Glicker, an introverted boy plagued by allergies. He confirms her suspicions that Debbie is the "Black Widow" and Fester is her next target.
At the end of the summer, the campers stage a play, A Turkey Named Brotherhood, about the first Thanksgiving. The "good" kids are cast as the pilgrims while the social outcasts are the Chippewa natives, with Wednesday as Pocahontas. When Wednesday, Pugsley, and Joel refuse to participate in the play, all three are locked in the Harmony Hut and forced to watch movies and TV shows ranging from Bambi to Lassie Come Home, The Little Mermaid, The Sound of Music, The Brady Bunch, and Annie. When the three come out, they pretend that they have changed from gloomy to cheerful; Wednesday even goes so far as to smile. During the play, however, Wednesday breaks out of character and makes a speech about how the American colonists will exploit the Native Americans in the future, then leads the other outcasts in revolt, destroying the set and tying Amanda up to be burned at the stake. Wednesday and Joel flee to the Chippewa fence as Pugsley steals a camp van. After hopping the fence, Wednesday asks Joel to come with them. He declines, saying he needs to go back to the others and "to watch the place burn". Realizing they may never see each other, the two young lovers exchange relatively emotional goodbyes and share a kiss through the chain-link fence. Afterward, the Addams siblings flee for home.
Meanwhile, Debbie has come up with a new plan to kill Uncle Fester. For their "three week anniversary", she wraps up a time bomb in a gift box and tells Uncle Fester not to open it until she comes back with some champagne. However, the bomb only succeeds in blowing up the house. When a frustrated Debbie snarls, "I want you dead, and I want your money!" Uncle Fester realizes the truth and flees with Thing's help to the Addams mansion, with Debbie in hot pursuit.
An ailing Gomez tells Uncle Fester that he is "Mr. Debbie!" but Fester declares, "I AM AN ADDAMS!" Gomez is instantly cured hearing this, and the baby reverts back to his dark, gloomy self. Pugsley and Wednesday return as well, but just then an angry Debbie bursts into the room to kill all the Addamses and take the money. She straps everyone except Pubert to electric chairs and uses slides to tell how she came to be. She killed her parents on her tenth birthday because they did not buy her the Ballerina Barbie doll she wanted. She killed her first husband, a heart surgeon, with an axe (reminding Granny Frump of old times) because he was too busy to dine with her (because of the Pope having a cold), and ran over her second husband, a senator, with her car because he would not buy her a new Mercedes-Benz (as they had "to set an example"). Uncle Fester begs Debbie to kill just him and let his family go, but she rejects his offer. Pubert short-circuits the wiring just as she throws the switch, saving the family and causing Debbie to electrocute herself.
Some time later, Gomez and Morticia throw a birthday party for Pubert. Joel, dressed like Gomez, sneaks off to the graveyard with Wednesday, where they talk briefly about Debbie. Remarking that Debbie's methods were "sloppy," Wednesday says that if she wanted to kill a man and not be caught, she would scare him to death. As Joel lays flowers on Debbie's grave, an unidentified hand breaks the surface and grabs his arm; Wednesday looks on, smiling, satisfied with Joel's screams.
- The Addams Cradle and Marie Antoinette appear.
- The tombstone of Florine Addams is shown.
- Strains of "The Addams Family: Main Theme" play throughout, but a new theme, "Addams Family (Whoomp!)", was composed for the film, and performed by Tag Team.
- Anjelica Huston as Morticia Addams
- Raul Julia as Gomez Addams
- Christopher Lloyd as Uncle Fester Addams
- Joan Cusack as Debbie Jellinsky Addams
- Christina Ricci as Wednesday Addams
- Carol Kane as Grandmama
- Jimmy Workman as Pugsley Addams
- Kaitlyn Hooper as Pubert Addams
- Kristen Hooper as Pubert Addams
- Carel Struycken as Lurch
- David Krumholtz as Joel Glicker
- Christopher Hart as Thing
- Dana Ivey as Margaret Addams
- Peter MacNicol as Gary Granger
- Christine Baranski as Becky Martin-Granger
- Mercedes McNab as Amanda Buckman
- Sam McMurray as Don Buckman
- Harriet Sansom Harris as Ellen Buckman
- Julie Halston as Mrs. Glicker
- Barry Sonnenfeld as Mr. Glicker
- Nathan Lane as Desk Sergeant
- John Franklin as Cousin Itt
- Charles Busch as Countess Cousin Aphasia du Berry
- Laura Esterman as Cousin Ophelia
- Maureen Sue Levin as Flora Amor
- Darlene Levin as Fauna Amor
- Carol Hankins as Dementia
- Steven M. Martin as Donald
- Douglas Brian Martin as Dexter
- Ryan Holihan as Lumpy Addams
- Lois DeBanzie as Delivery Nurse
- Vickilyn Reynolds as Forceps Nurse
- Cynthia Nixon as Heather
- Edye Byrde as Mrs. Montgomery
- David Hyde Pierce as Delivery Room Doctor
- Andreana Weiner as Obnoxious Girl
- Peter Graves as Host
- Rick Scarry as Lawyer
- Monet Mazur as Flirting Woman
- Francis Coady as Flirting Man
- Ian Abercrombie as Driver
- Chris Ellis as Moving Man
- Camille Saviola as Concetta
- Zack Phifer as Passport Clerk
- Tony Shalhoub as Jorge
- Jeffrey Van Hoose as Irwin
- Micah Winkelspecht as Mordecai
- Matthew Beebe as Wheelchair Camper
- Kristy Shirvani as Consuela
- Jamie Gordon as Esther
- Micah Hata as Yang
- Joey Wilcots as Jamal
- Jason Fife as Camper #1
- Karl David-Djerf as Camper #2
- Haley Peel as Young Debbie
- Cheryl Chase as Pubert (voice) (uncredited)
- Matthew Bartilson as Dancing Camper (uncredited)
- Alex Gaona as Indian Camper (uncredited)
- Nichole McAuley as Snobby Mom (uncredited)
- Rob Sanchez as Sailor (uncredited)
- Dianna Deuce as Puppet Nanny (uncredited)