As the final vestiges of Rudolf Smuntz's empire collapse, his two sons, Lars and Ernie, find themselves at a pivotal moment in their lives. The responsible and hopeful Lars has always been committed to the family string factory, whereas Ernie, a self-centered cynic, seeks to forge his own path in the culinary world. However, his aspirations crumble when an unfortunate incident involving a rogue cockroach—a remnant from his late father's cigar box—leads to the mayor’s heart attack. This calamity leaves Ernie jobless, his culinary dreams in ruins, and ultimately results in his descent into homelessness.
At the same time, Lars grapples with a crucial decision: should he accept the enticing buyout offer from Zeppco International for the factory, or adhere to his father’s dying wish to keep the family business alive? When his gold-digging wife, April, leaves him after he turns down the lucrative deal, Lars is left alone in the decaying Victorian mansion that he and Ernie inherited from their father.
The brothers endure a torturous night within the mansion, plagued by the endless scratching of a cunning mouse. Their curiosity leads them to uncover hidden treasures: the mansion was the crowning achievement of famous architect Charles Lyle LaRue. With the potential for immense restoration value, Lars and Ernie seize the chance to reclaim their fortunes by renovating and auctioning off their inheritance. Yet, Ernie’s paranoia over the rogue cockroach encourages him to advocate for the removal of the mouse as well. Their traditional attempts to rid themselves of the clever rodent falter, as it seems to possess an uncanny sense of humor. The brothers are soon resorting to extreme tactics, which include acquiring an enormous Maine Coon cat dubbed "Catzilla," along with hiring the eccentric exterminator, Caesar.
Their efforts to eliminate the mouse quickly descend into chaos: Catzilla meets an untimely end in the dumbwaiter, while Caesar finds himself comically dragged through the mansion corridors by the mischievous rodent, who has commandeered his truck's winch line. In this pandemonium, the brothers face the reality of their absurd predicament and the complex dynamics of their relationship.
As Ernie's financial troubles escalate, the risk of foreclosure creeps closer, particularly due to the mortgage he took out for the mansion's extravagant renovations. The bank issues an ultimatum—gather the funds within two days or face eviction—triggering a series of upheavals in the once-quiet household. Their dwindling resources necessitate a layoff of their workers, causing a strike that threatens their renovation project. In a moment of desperation, Ernie contemplates accepting Zeppco’s enticing buyout offer and arranges a covert meeting to finalize the deal. Meanwhile, Lars is motivated to save their fortunes by producing string in order to settle the mortgage, only to find himself thwarted by April, who has come to recognize the mansion's true worth and desires to claim it for herself.
As Ernie’s meeting with Zeppco spirals out of control—thanks to his distractions involving a group of beauties and an unfortunate bus accident—the brothers' already tense relationship reaches its breaking point. Back at the mansion, Lars accidentally uncovers a message from Zeppco on their answering machine that lays bare Ernie's betrayal, igniting a heated dispute between the siblings. The atmosphere thickens with tension as they confront each other, while Caesar lurks in the background, a silent witness to their turmoil.
Their harried attempts to rid themselves of the bothersome mouse backfire spectacularly, triggering a bug bomb left by Caesar that creates a massive hole in the floor, plunging their renovation efforts into disarray. As the brothers’ animosity towards one another grows, so do the stakes, culminating in an explosive clash that leaves the pesky mouse dazed but still very much alive.
As the auction day approaches, Lars finds that a postal box has returned to the mansion, its contents ravaged by the same mouse he once tried to eliminate. Meanwhile, Ernie has the surreal experience of addressing the auctioneers while the mouse casually occupies his podium. In one last-ditch effort to salvage their dreams, the brothers turn a garden hose on the mouse, inadvertently flooding an inner wall of the mansion, leading to an explosive collapse that sends auctioneers running and the building down in ruins.
In the wake of this catastrophe, April departs with her wealthy bidder, leaving the brothers to contemplate the debris of their once-grand inheritance. Although they find themselves stripped of everything, they gain an unexpected peace in the knowledge that Caesar's fate is now intertwining with the ruins of what was once their home.
As night falls on the forsaken factory, the weary brothers unwittingly stumble into a newfound tranquility amidst the crumbling façade. Unbeknownst to them, their newfound shelter becomes a playground for a perceptive little mouse. This mouse, sympathizing with the brothers' plight, awakens the factory’s dormant machinery and ingeniously combines ingredients to create an extraordinary giant ball of string cheese, a whimsical marriage of wax and cheddar. This moment of serendipity inspires a renewed vision for the brothers, who now see potential in this decrepit factory. With Lars taking the lead and Ernie embracing his role as head chef, alongside their tiny companion orchestrating tastings for their inventive cheese recipes, the factory transforms into a thriving center of dairy wonders, its air filled with the sweet scent of success and the enduring echoes of the mouse's clever involvement.
