After having an especially bad Monday, Garfield invents a mysterious machine, much to the concern of Jon, Herman, Vito, and Nermal.
Odie wants to play fetch with Garfield, which Garfield angrily refuses to do. Jon explains to Odie how Garfield's day was, and why he's in a bad mood. The day is Monday, and what has happened was bad, even for Monday standards.
First, while waiting to prank the mailman, Garfield discovered Herman had obtained a massive tank, which came with a retracble claw allowing him to deliver the mail from a distance, to prevent Garfield from pranking him anymore. Later, when he went to Vito's to eat, Garfield learned that he was not allowed back in until he payed for the last 7,432 pizzas he ate there. Then when he got home Garfield was shocked to discover that Nermal is staying at his home for months (with numerous suitcases). To make matters worse, Jon tells Odie that he has to tell Garfield that he is going on a diet.
After Jon tells Garfield about the diet, Garfield leaves the house, goes to the city dump, and comes back with some junk. He goes into the garage, builds a mysterious machine, and takes a nap on it. Jon, Herman, Vito, and Nermal all wonder what it does.
Herman thinks the machine is a giant magnet that will rip off the top of his tank and allow Garfield to prank him.
Vito thinks Garfield has invented a machine that makes the perfect pizza and believes that Garfield will start selling his own pizza and ruin Vito's business.
Nermal thinks the machine will strap him to a rocket and send him to Alpha Centauri.
Nermal packs up his stuff and tells Garfield he's leaving. Herman then appears and says he got rid of the tank and even falls into the mud. Vito shows up with pizza and erases Garfield's tab. Then Jon tells Garfield he has taken him off his diet.
Garfield and Odie are then eating the pizza that Vito brought. Garfield finally reveals that the machine doesn't do anything at all.