Rudolph Smuntz (the father of Ernie and Lars Smuntz) has died, leaving behind a famous albeit run-down string factory and an equally run-down old house, both to his two sons. Ernie Smuntz owns a famous restaurant and sees no point in running the string factory, but Lars, who has no job, thinks it would be a great opportunity to continue running "Smuntz String" for their father's sake. When Ernie accidentally kills the mayor in his restaurant, he is left with nothing and is forced to close down. With Ernie's restaurant and "Smuntz String" not doing so well, they decide to check out the house they were left. After finding some ancient plans for the house in the attic, they discover that the house is historically significant and very valuable. The brothers then decide to have the house restored in order to sell it -- but someone's out there to stop them: a mouse! And this is no ordinary mouse. It does everything to stop the renovation of the house and scare the Smuntz brothers away. The brothers fight back against the mouse, and when they think they have got rid of it they plan an auction at the house to sell it -- but the mouse returns and causes all sorts of havoc.
In the end, the ancient house collapses, leaving Lars and Ernie with nothing, again. Lars and Ernie return to the factory, thinking they have got rid of the the mouse, but they were wrong. The mouse returns to the factory with them and takes over the factory controls, creating a revolutionary idea which makes the brother rich..