At a science fair, Wednesday Addams is set to present her experiment, where she uses her pet squid Socrates’ DNA on Uncle Fester to show how humans can be improved. However, she is dismayed when she sees her family show up. Nonetheless, her work gets noticed by scientist Cyrus Strange. Back at the Addamses’ home, Gomez worries that the children are drifting apart from him and Morticia, so he decides to take them on a family vacation. Before the family, Fester, Thing, and Lurch head on a road trip across the country, Gomez and Morticia are approached by a lawyer named Mr. Mustela; he claims that Wednesday was switched at birth and may not actually be an Addams, but they ignore him. On the road, the Addams family is pursued by Mustela and his henchman Pongo.
The family is initially set on going to Salem, Massachusetts, but Fester ends up making a detour to Niagara Falls. The family later stops in Sleepy Hollow, New York. Gomez and Morticia bring up Mustela and his claims about Wednesday, so Fester mentions a story from the day Wednesday was born in where he juggled babies in a delivery room. This only worsens Gomez and Morticia’s fears that Wednesday was switched, which Wednesday overhears.
Gomez brings everyone to Miami to get in touch with Cousin Itt, in hopes he can help with their current dilemma. Itt joins them on the trip as they pass through San Antonio and leaves the family early at the Grand Canyon. Meanwhile, Wednesday traps Mustela on her own and learns he is working for Cyrus. He suggests to Wednesday that he is her true father and invites her to his home in Sausalito, California. Wednesday runs a DNA test using Gomez’s hair, which seemingly proves that she most certainly is not his daughter. Wednesday leaves the family while they sleep, but Lurch catches up with her. The rest of the Addamses learn where she is heading and follow suit.