The Monster Squad is an adventurous group of pre-teens who are passionate fans of classic Universal monster films. The leader, Sean Crenshaw (Tom Noonan), finds himself in a unique predicament when he receives a mysterious diary belonging to the renowned monster hunter, Dr. Abraham Van Helsing. Although he is initially thrilled, Sean's excitement swiftly vanishes upon realizing the diary is in German. Determined to decipher its contents, he, along with his trusty squad—his best friend and right-hand man Patrick Rhodes (Andre Gower), the bumbling yet lovable Horace (Ryan Lambert), the tough older kid Rudy (Duncan Regehr), and the youngest member Eugene (Michael Faustino)—seek out a local recluse known as the "Scary German Guy" (Mary Ellen Trainor), who turns out to be a gentle soul with a troubled past.
The diary reveals the existence of a powerful amulet made of concentrated good, which can be destroyed only once a century when the cosmic balance of good and evil teeter toward equilibrium. With this rare alignment approaching at midnight in just a few days, the squad realizes their urgent mission: to capture the amulet and use an incantation from Van Helsing's diary to create a wormhole that can banish monsters to Limbo. As the backstory unfolds, we learn that Van Helsing tried and failed a century ago to defeat the dreadful Count Dracula; his followers managed to smuggle the amulet to America for safekeeping from Dracula's relentless pursuit.
However, the evil Dracula has other ideas and gathers a monstrous ensemble which includes the Mummy, the Gill-man, and the Wolf Man, along with his newly transformed vampiric consorts—three unsuspecting schoolgirls. In a daring move, he commandeers a crate containing Frankenstein's monster from a plane, completing his sinister lineup. Yet, Frankenstein's monster turns out to be gentle-hearted, roaming into the woods where he meets Phoebe (Ashley Bank), Sean's little sister. Instead of fear, Phoebe extends her kindness, and they quickly form a friendship. After witnessing his good nature, the Monster Squad convinces Frankenstein’s monster to join them in their fight against Dracula.
The amulet is hidden beneath a house currently occupied by Dracula and his fearsome cohorts, secured by wards that prevent easy access. As they execute a thrilling break-in, the squad successfully retrieves the amulet but narrowly escapes the clutches of the vampires. They learn from "Scary German Guy" that they need a virgin to read the incantation. In retaliation for their meddling, Dracula destroys their clubhouse, drawing in Sean's father, Detective Del Crenshaw (Stephen Macht), into their chaotic battle against the forces of darkness.
As midnight looms closer, the Monster Squad races to a cathedral for their final stand against the approaching evils. When they discover that the cathedral doors are locked, they resort to performing the incantation on the steps, which leaves them open to attack. In a desperate bid for help, they turn to Lisa, Patrick's sister, who they believe is a virgin, but the attempt fails spectacularly as her true status is revealed. As monsters close in, the squad deduces that only Phoebe can read the incantation, and with help from "Scary German Guy," she prepares to summon the portal while the rest fend off Dracula and his horrifying minions.
In an epic showdown, Dracula's allies are vanquished one by one. Just as the tides seem to turn, Dracula arrives to obliterate the amulet, but Frankenstein's monster intervenes heroically, impaling him upon a cross. At the critical moment, Phoebe completes the incantation, unleashing a vortex that threatens to engulf the monsters. In a gripping climax, as Dracula attempts to pull Sean into the abyss with him, Sean stakes Dracula, while Detective Crenshaw saves him from falling in. Van Helsing makes a brief appearance, ensuring Dracula meets his demise. As the portal consumes the monstrous entities, Phoebe desperately clings to Frankenstein's monster, who ultimately sacrifices his chance to remain, accepting only a cherished stuffed animal from her as a memento.
With the threat eliminated, the United States Army arrives, responding to Eugene's earlier plea for help. Amid the chaos, when the bewildered general fails to grasp the situation, Sean takes the initiative to step forward and introduces himself and his friends with flair, declaring, "We are The Monster Squad."
