The film follows two Laurel and Hardy-like brothers in their struggle against one small but crafty house mouse for possession of a mansion which was willed to them by their father.

When the once-wealthy string magnate Rudolf Smuntz dies, he leaves his factory and a dilapidated Victorian mansion to his two sons; the dutiful and optimistic Lars, and venal cynic Ernie, who has ignored the family business to become a chef; he walks out of the reading of their father's will, taking a box of cigars.

At Ernie's restaurant, a cockroach crawls out of the box of cigars and into a dish prepared for the Mayor McKrinkle, causing him to get fatally heart attacked when he accidentally ate one and is taken away in an ambulance where they vainly try to revive him, but fail, much to his wife's sorrow while the kids play with the sticky signs.

Meanwhile, a cord company called Zeppco International offers Lars a buyout for the string factory, but then remembers his promise to his father never to sell it, and refuses.

The blueprints reveal that the mansion was the final design of the famous architect, Charles Lyle LaRue, and would be worth a fortune if restored.

The brothers resort to extreme measures to remove the mouse, including buying a monstrous Maine Coon cat named "Catzilla" and hiring an eccentric exterminator named Caesar; the mouse drops Catzilla to his death in a dumbwaiter, and traumatizes Caesar by dragging him around the mansion using his truck's winch line.

Ernie chases the mouse with a shotgun and accidentally shoots a bug bomb Caesar had dropped, blowing a massive hole in the floor and sending him and Lars tumbling.

On the night of the auction, Lars discovers the postal box returned due to insufficient postage and a hole chewed in it, while Ernie sees the mouse on his podium as he gives a speech.

As the auction begins, the brothers attempt to flush the mouse out with a garden hose, filling an inner wall of the mansion with water until it bursts, washing the bidders out and causing the estate to collapse.

When April leaves with a wealthy bidder, the brothers are now left with nothing, but take solace in that the collapse must have indeed finally killed the mouse.

Lars (now having formed a relationship with a Belgian model named Hilde) runs the factory with Ernie as the chef and the mouse as their taste-tester for new cheese combinations.