As Gomez (Raul Julia) and Morticia (Anjelica Huston) Addams celebrate the arrival of their newborn son, Pubert, they enlist the help of Debbie Jellinsky, a seemingly benign nanny, to assist them with childcare. However, their delight is overshadowed by the ulterior motives of Debbie, who harbors a sinister secret: she is a cold-blooded serial killer who lures in wealthy bachelors, marrying them only to murder them and seize their fortunes. As Debbie cunningly integrates herself into the Addams family dynamics, she deceives Gomez and Morticia into believing that their children, Wednesday and Pugsley, are eager to attend summer camp—when in truth, the siblings are desperate to remain at home.
At Camp Chippewa, a perky summer retreat overseen by the overly cheery Gary and Becky Granger, Wednesday and Pugsley experience alienation from their campmates due to their unique tastes and styles. In this environment of forced joviality, they forge an unexpected friendship with Joel Glicker, a bookish outcast who harbors a crush on Wednesday. Meanwhile, Debbie continues to manipulate Gomez and Morticia into sending the twins away, leading her to an unexpected romance with Uncle Fester (Christopher Lloyd).
As the wedding of Debbie and Fester approaches, the tension in the air is thick with Debbie's disdain for the Addams family, although Uncle Fester's passionate vows seem to draw them closer together. During their honeymoon, however, tensions rise when Debbie tries to rid herself of Fester in a bathtub, but once again her plans go awry. As she concocts further schemes to drive a wedge between Fester and the Addams clan, Gomez is left heartbroken over what he perceives as his son's disappointing transformation into a conventional young boy with blonde hair and blue eyes.
Back at Camp Chippewa, Wednesday is coerced into playing Pocahontas in Gary's Thanksgiving pageant titled "A Turkey Named Brotherhood," igniting her rebellious spirit. As punishment for her defiance, she, Pugsley, and Joel find themselves exiled to the Harmony Hut, where they're subjected to relentless feel-good movies. This forced atmosphere of cheer soon becomes the catalyst for a plan of revenge as they unite with other outcasts against their cheerful tormentors. Wednesday's return to her authentic self is marked by a heartfelt gesture from Joel, solidifying their bond as they plot to dismantle the camp once and for all. When the dust settles, the siblings escape in a stolen camp van, their connection stronger than ever before.
As Debbie's nefarious ambitions are revealed, she detonates an explosion in the Addams mansion, but Fester emerges miraculously unscathed from the wreckage. The depth of her obsession becomes clear as she pulls a gun and confesses her greed-fueled love for Fester's wealth. Cleverly, Thing intervenes, knocking Debbie out of the way with her own vehicle, allowing Fester to reunite with Gomez in their ancestral home. The Addams family enjoys a fleeting moment of unity before Debbie resurfaces, armed and intent on electrocuting them in their electric chairs. As she shares her gruesome backstory—detailing the murders of her parents and past husbands—the family listens, torn between horror and empathy.
In a miraculous turn of events, Pubert, restored to his original state, escapes from his crib and navigates through a series of improbable mishaps to reach his family. Just as Debbie prepares to strike the final blow, Pubert reroutes the electrical current back to her, turning her body to ash and saving his family from a gruesome fate.
In the aftermath of their harrowing ordeal, the Addamses gather to celebrate Pubert's first birthday, with Joel in attendance. Fester, though mourning Debbie's demise, swiftly finds his heart captivated by the new nanny, Dementia, brought in by Cousin Itt and his wife, Margaret Alford. As the family honors Debbie's memory at the cemetery, Wednesday expresses her views on Debbie's inept methods, sarcastically noting that she would have terrified her victims into silence rather than leave a trail of evidence.
As Joel lays flowers at Debbie's grave, an eerie hand bursts through the soil, grasping him and pulling him into the darkness—a chilling reminder that the Addams legend is far from over.
