Garfield, suddenly feeling depressed, believes he has not made a difference in the world. To cheer him up, Paddy the Leprechaun teaches Garfield about his importance.
On a rainy day, Garfield is feeling depressed and more gloomy than usual. Jon tells Liz that he is worried about how Garfield has been acting lately, so Liz suggests trying to find a way to cheer him up. To make Garfield feel better, Jon tells Garfield that he will be making sausage lasagna, which Garfield is not enthusiastic about hearing.
Garfield is depressed because of how unexciting his life is and thinks that he has not made a difference in the world because all he does is eat, sleep, and throw the stick for Odie to fetch. He then decides to go for a walk as the rain lets up. When the rain stops, Garfield notices a rainbow and decides to follow it. On the other side is Paddy the Leprechaun. Garfield explains to Paddy that he is feeling useless and wishes he never existed, so Paddy decides to help him see how important he truly is.
Paddy shows Garfield shadows of a world where he does not exist, with all his friends apparently having better lives without him; Jon and Odie are fabulously wealthy since Jon does not have to pay for feeding Garfield, Arlene and Bruno are happily in love, and Odie has a butler to play fetch with him.
Upon seeing his friends' lives without him, Garfield believes that he truly is useless, until Paddy shows him that the lasagna aliens from Planet Parma are invading Earth because there is no one to defend the planet from them, since nobody loves lasagna as much as Garfield. Paddy briefly makes Garfield visible to the rest of the universe so he can stop the aliens, then makes him invisible again after he stops the invasion.
Despite saving the alternate universe, Garfield still feels useless, so Paddy takes him back to his friends to see how they truly feel without him. First, he shows him that Jon and Odie are bored and lonely without another member of their family. Although they try to adopt a cat, there is no cat that could make their lives more interesting and fun. Then, Paddy shows Garfield that Arlene and Bruno broke up because Arlene wants a boyfriend who is unique with a dry sense of humor and likes lasagna, which Garfield is. Next, Paddy then shows Garfield that Vito’s pizza place is out of business because Garfield’s insane gluttony is what kept the store in business. Paddy finally shows Garfield the most important reason for his existence: his fans. If he did not exist, his fans would be completely bored and disappointed because he was not around.
Realizing how truly meaningful his life is, Paddy returns Garfield home with a happy and grateful attitude. He starts playing fetch with Odie and promises Arlene to be on time for their date. He tells them how much he matters in life and then tells the viewer that they matter as well, and that they do not need a leprechaun to tell them that.