The story begins at a middle school science fair, where Wednesday Addams, a bright and inventive young girl, is excited to present her latest project. However, her enthusiasm quickly diminishes when her eccentric family members — Gomez Addams, Morticia Addams, and Pugsley Addams — make a sudden entrance, diverting the crowd's attention away from her. Undaunted, she showcases her experimentation involving squid DNA integrated into Uncle Fester's physiology, which could enhance human abilities. Though she receives applause for her work, her hopes of claiming first place are dashed when she learns that everyone received a trophy, making her triumph feel trivial. Amidst this letdown, she attracts the gaze of the mysterious scientist Cyrus Strange, who praises her innovation through a holographic projection.
Meanwhile, Pugsley mistakenly tries to impress his classmates by tampering with another student's volcano project, resulting in an explosive mishap that creates chaos in the gym. Back at their home, both Wednesday and Pugsley decline dinner invitations, prompting Uncle Fester to help Pugsley with his social skills. Wednesday writes a cryptic note regarding her absence, which worries Gomez about the increasing distance between him and his children. In response, he and Morticia decide to organize a family vacation.
Gomez unveils a macabre vehicle for their journey, and the entire Addams family, including Thing, Fester, the children, and Lurch, embark on the adventure, leaving Grandmama behind to manage the estate. Just before departure, a lawyer named Mr. Mustela approaches them with an alarming claim: Wednesday might have been switched at birth, calling into question her identity as an Addams. Gomez and Morticia quickly dismiss the suggestion, interpreting it as Wednesday's latest ploy to avoid the family trip, yet she insists that she knows nothing about the mysterious stranger. As they hit the road, Mustela and his menacing henchman, Pongo, follow them with ill intentions.
As Uncle Fester becomes increasingly obsessed with water, the family's initial destination shifts from Salem to the breathtaking Niagara Falls. This unexpected stop becomes significant when Pugsley attempts a cheesy pickup line on some girls, only to be left embarrassed. Meanwhile, Wednesday seeks revenge by using a voodoo doll of her brother to orchestrate a dance routine, resulting in a memorable toss into the Falls’ mist. When Gomez and Morticia try to connect with their increasingly distant daughter, she rejects their advances, becoming engrossed in her quest to uncover the truth about her origins.
Their journey takes a sinister turn as they settle in Sleepy Hollow for a night filled with spooky campfire stories. While Gomez and Morticia contemplate Mr. Mustela's assertions regarding Wednesday's identity, Fester entertains them with a bizarre tale about her birth. He claims that he broke into the delivery room and used a peculiar Addams family tradition of juggling newborns to help them sleep, only amplifying Gomez and Morticia's fears about their daughter's true lineage.
Listening to her parents’ worries, Wednesday confides in Lurch about her suspicions surrounding her parentage. As their adventurous trip continues, Gomez devises a plan to visit Cousin Itt (played by Snoop Dogg) in Miami, hopeful that Itt's wisdom can shed light on their predicament. The mysterious cousin joins them on their travels through San Antonio, where he shares tales of rebellion with Wednesday. However, just as they start to reconnect, Grandmama calls Itt back to control a chaotic house party.
A twist of fate occurs when Lurch's quick thinking saves the family from Mustela and Pongo, who inadvertently crash into a giant pile of manure. Seeking sanctuary from the relentless gaze of Mustela and Pongo, the Addams family finds refuge in a motel where Wednesday is begrudgingly entered into a tacky girls' beauty pageant. Visibly displeased, she uses her telepathy to scare one contestant and sabotages the event by drenching the other girls in bright red paint, reminiscent of "Carrie." As Mustela nearly recognizes her disguise, Gomez diverts attention by impersonating a flamboyant talent agent, allowing the family to flee the scene amid the chaos. Meanwhile, Lurch and Pongo share a significant look as they blend into the crowd.
Their adventure leads them next to the stunning Grand Canyon, where Uncle Itt mysteriously disappears, prompting Morticia to gift Wednesday a necklace symbolizing their family's lineage. Ignoring the moment, Pugsley causes a ruckus by detonating an explosive, drawing unwelcome attention once more. Fester's transformation into a hybrid human-octopus continues as their journey unfolds.
However, Wednesday's solo exploration leads her face-to-face with Mustela, who has been on her tail. Unfazed, she outsmarts him and uncovers that he was only a puppet in Cyrus Strange's scheme, who claims to be her real father and invites her to join him in Sausalito to develop her scientific abilities.
Choosing to leave her family while they slumber, Wednesday is confronted by Lurch, who takes on the protector role. Once the rest of the family learns of her whereabouts, they pursue her. Stopping at a lively biker bar, Wednesday enlists the bikers’ help to reach Sausalito after Lurch impressively plays Gloria Gaynor’s "I Will Survive" on the piano. The bikers then help them reach their destination.
Upon arriving at Cyrus's headquarters, Wednesday meets his quirky crew, including Ingrid, a bird-like humanoid, and their shape-shifting "daughter" Ophelia. Cyrus presents her with a formula alarmingly similar to her own experiments aimed at creating human-animal hybrids. As the rest of the Addams family arrives, Wednesday's indifference becomes apparent, suggesting she may align more with Cyrus than her family. Meanwhile, Gomez and Morticia grieve the emotional distance from their daughter, while Pugsley learns Ophelia is just a pig in disguise.
Cyrus ultimately reveals his true motives — he seeks to exploit Wednesday's formula for his nefarious research. Utilizing manipulation, he ensnares the family in traps. However, as Wednesday witnesses her family’s predicament, she is compelled to take action against Cyrus. Just as she is about to confront him, an unexpected alliance forms between Lurch and Pongo, who share a past from their time at the asylum. Pongo warns Lurch, allowing them to team up and free the Addams family.
With Cyrus caught in one of his own experiments, he transforms into a monstrous hybrid, wreaking havoc and battling with Fester, who has morphed into a colossal squid-like entity. Their clash culminates in both tumbling out of the lab window. Cyrus meets a fitting end, but Fester’s transformation leaves him in a weakened state.
In a poignant moment, Wednesday uses blood from Morticia's necklace to revive Fester. As he returns to his human form, Wednesday finds comfort in her family once again, embracing her identity as an Addams fully. Their thrilling escapade concludes with Ophelia reemerging, now a solid presence in their family thanks to Itt's charm.
As this unique adventure wraps up, Wednesday confronts an unsettling truth: despite Cyrus's claims that he forged the DNA results, her own test revealed she shares no resemblance to Gomez. The truth begins to unravel when Cyrus sheepishly admits that while he may be concealing his toupee, he confesses that Wednesday is undeniably his biological daughter. With this shocking revelation, the Addams family embarks on yet another eccentric journey, preparing for their next wild exploits around the globe while navigating the complexities of their wonderfully strange dynamics.
