Conventional methods fail when the mouse demonstrates itself to be exceptionally intelligent.

Ernie, fearing a repeat of the cockroach incident, convinces Lars they must also get rid of the mouse. The brothers decide to renovate and auction the mansion to recover their lives. The blueprints reveal that the mansion was the final design of a famous architect, Charles Lyle LaRue, and it would be worth a fortune if restored. The brothers cannot sleep due to noises caused by a mouse, and while investigating find blueprints of the property. With nowhere else to go, the brothers spend the night in the mansion. Lars's wife April is furious and kicks him out. Meanwhile, a cord company called Zeppco International offers Lars a buyout for the string factory, but doesn't notice that he remembers he promised his father to never sell it, and refuses. Shortly after the mayor dies, Ernie's restaurant, Chez Ernie, is condemned and scheduled for demolition, costing Ernie his career and home in the process. At Ernie's restaurant, a cockroach crawls out of the box of cigars and into a dish prepared for the mayor, causing him to have a heart attack when he accidentally bites into it and is taken away in an ambulance.

When the once-wealthy string magnate Rudolf Smuntz dies, he leaves his factory and an abandoned 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.

Mouse house movie