Young filmmaker and scientist Victor Frankenstein (Charlie Tahan](/actor/charlie-tahan)) resides with his parents, Ben and Susan Frankenstein (Martin Short and Catherine O'Hara), alongside his beloved dog Sparky, in the serene town of New Holland. Victor’s intelligence is notable among his schoolmates, including his gloomy next-door neighbor Elsa Van Helsing (Winona Ryder), the mischievous and egregious Edgar "E" Gore (Atticus Shaffer), the portly and naive Bob (Robert Capron), the overconfident Toshiaki (James Hiroyuki Liao), the eerie Nassor (also Martin Short), and a peculiar girl often referred to as Weird Girl (also Catherine O'Hara). Although intelligent, Victor struggles to connect with his peers, largely due to his devotion to Sparky. Concerned about his son's social isolation, Victor's father urges him to participate in baseball to achieve balance beyond his scientific interests. Although Victor impresses everyone by hitting a home run during his first game, tragedy strikes when Sparky is tragically killed by a car while chasing a ball.
Grieved yet inspired by a demonstration from his science teacher Mr. Rzykruski (Martin Landau), who illustrates the effect of electricity on deceased frogs, Victor makes a bold decision. In a desperate attempt to revive his beloved dog, he digs up Sparky's body, transports him to his makeshift attic laboratory, and, utilizing lightning, successfully brings him back to life. However, Sparky’s adventure begins when he escapes into the neighborhood after catching the eye of Weird Girl's living cat, Mr. Whiskers. This prompts Edgar to blackmail Victor into teaching him the secrets of reanimation. Their efforts culminate in the accidental revival of a dead goldfish, which subsequently turns invisible due to an experiment gone awry. Edgar, in a flurry of excitement, boasts about the invisible creature to Toshiaki and Bob, leading to a series of reckless science fair preparations that endanger everyone involved. This results in Bob injuring his arm and consequences for Mr. Rzykruski, who finds himself wrongly accused and dismissed.
As events unfold, Edgar’s claims lead to chaos, particularly when the invisible fish's secret is discovered, inciting Toshiaki, Nassor, and Bob to attempt their own reanimations. In a panic, Victor's parents discover Sparky in the attic, leading to a chase that sends Sparky fleeing. Meanwhile, the classmates, without their mentor's guidance, begin conducting their own experiments with catastrophic results, transforming innocent animals into monstrous beings. Critically, Mr. Whiskers holds a bat while it's electrocuted, turning it into a vampire cat; Edgar resurrects a rat from the garbage—a creature that subsequently mutates into a wererat; Nassor’s mummified hamster, Colossus, is revived; Toshiaki's turtle Shelley grows into a giant monster; and, finally, Bob’s Sea-Monkeys turn into amphibious horrors. These creatures raid the local fair, causing chaos and panic.
In the midst of the turmoil, Victor encounters Sparky at the pet cemetery and races to assist his classmates against the monstrous onslaught. The mayhem escalates when the Sea-Monkeys meet their demise after consuming salt-covered popcorn, while Colossus faces an unfortunate fate under Shelley’s foot. As the monsters wreak havoc, Elsa, the mayor's niece, is snatched by Mr. Whiskers and taken to the town windmill. In an unfortunate twist, the townsfolk mistakenly place the blame on Sparky, prompting a wild chase that leads them to the windmill. An accidental ignition by Mayor Bergermeister (also Martin Short) creates a fiery crisis. In a heroic moment, Victor and Sparky plunge into the inferno to save Elsa, but Victor becomes trapped, and Sparky courageously attempts a rescue, only to be caught by Mr. Whiskers.
In an intense confrontation, as Mr. Whiskers corners Sparky, a flaming piece of wood pierces him fatally. The windmill ultimately collapses onto Sparky, seemingly ending his life once more. However, in a heartwarming turn of events, the townspeople unite to revive Sparky using car batteries, eternally grateful for his bravery. The story concludes as Persephone, Elsa's poodle with a hairstyle reminiscent of the iconic Bride of Frankenstein, approaches Sparky, hinting at a blossoming love between the two dogs.
