As Jesse McLaughlin (Jesse) and his girlfriend Kate settle into their new life within the ancient walls of the family mansion, they welcome Jesse's old friend Charlie Coriell, who arrives with his glamorous partner Lana in an effort to impress Kate. Little do they know that Charlie's visit triggers a series of events that will unveil the secrets hidden beneath the mansion's stone façade.
While exploring the depths of the basement, Jesse discovers a dusty photo of his great-great-grandfather standing beside an Aztec temple, proudly holding a dazzling crystal skull encrusted with sapphires. The photograph also features Slim Reeser, a once-loyal companion turned enemy after a conflict over the prized relic.
Unaware of the skull's burial location, Jesse and Charlie devise a scheme to unearth their ancestor's remains, hoping to unearth the legendary artifact. However, their actions disrupt the resting place of Jesse's great-great-grandfather, who miraculously awakens as a warm-hearted "Gramps."
As Gramps shares thrilling tales of his Wild West adventures with Jesse and Charlie, he reveals that the mansion was built using stones from the Aztec temple, and its rooms function as a time-travel portal, with the skull as the key to this mystical doorway.
Guided by Gramps, the trio learns of the dark forces intent on seizing the skull, which they must protect at all costs. Meanwhile, tensions rise as Kate feels disconnected from Jesse, especially as he reconnects with an old flame. A Halloween party hosted by Charlie serves as a pivotal moment for Gramps to make an unexpected appearance.
Compounding the mix of emotions is the insidious influence of John Statman, causing Kate to leave Jesse and Lana behind. As the night unfolds, Jesse, Charlie, and their quirky new friends—a baby pterodactyl and a caterpillar-dog called the Cater-Puppy—face off against a Jurassic-era barbarian who has stolen the coveted skull.
Bill Towner, an unassuming electrician yet a seasoned adventurer, comes to inspect the old home's electrics. Initially portrayed as a lovable fool, he surprises everyone by producing a tiny sword from his tool belt, leading Charlie and Jesse through a hidden portal in the house. They land in ancient Aztec territory, thrust into an exhilarating battle against fearsome warriors as they strive to save an innocent virgin from a sacrificial fate.
In this mystical land, a revived Slim Reeser, fixated on the skull, confronts them in a thrilling showdown. His relentless pursuit culminates in a violent encounter with Jesse's grandfather, who, while wounded, reveals that Slim was responsible for Jesse's parents' tragic deaths when he was just a baby. This shocking revelation propels Jesse into a vengeful spree, culminating in a defiant leap through a window into the Wild West—where he exacts justice by shooting Slim's undead form.
As Gramps succumbs to his injuries, he shares a touching farewell with Jesse, expressing immense happiness at having finally met his great-great-grandson. In his final moments, Gramps warns Jesse about the skull's immense power, urging him to embrace its mysteries and ultimately destroy the cursed object. As Gramps' spirit departs, Jesse holds his grandfather in a heartfelt embrace.
The film concludes with an exhilarating twist: empowered by the crystal skull, Jesse has journeyed back in time to the Old West, embarking on a new frontier adventure alongside Charlie and their quirky companions, leaving the enchanted relic as a testament to Gramps’ everlasting legacy.
