Dennis and Dee's abrasive mother has died, and the bad news is that she stiffed them in her will, leaving all her money to her humanitarian paramour, Bruce Mathis. But at least Dennis got her house. It's a "party mansion," and that's how he intends to use it.

Dee gets a rude awakening when she discovers that "Fatty Magoo," an overweight loser from her high-school class, is now a svelte, successful clothing-store owner. So Dee decides to design a dress for her, but Dennis decides that he's the better designer. He also knows that he has the charm to sell it to Fatty.