The plot is based on Mark Twain's The Prince and the Pauper: Garfield travels to London and manages to trade roles with a rich, English, fat, orange cat named Prince.
When Jon (Breckin Meyer) learns that his love, Liz, needs to go to London, Jon packs his bags and goes to England, trying to pleasantly surprise Liz. Odie and Garfield (Bill Murray) are put in a kennel, but they somehow break out and empty Jon's suitcase to make room for them. Soon they end up lost in the Big City, as they set out to find Jon, but they also anger a Royal Guard by peeing on his leg!
Meanwhile, Prince (Tim Curry), a cat, has just been selected as the successor of Lady Elanor's estate. Dargis, who is now angry that the cat has inherited his own estate, attempts to get rid of Prince. Prince is thrown into the river, ends up in the London Sewers, and is found by Jon while Garfield is mistaken as Prince and is picked up by Smithee the Servant.
Soon, Dargis attempts to kill the cat and sets his own dog, Rommel, on him. Thinking that the cat is now dead, he invites the solicitors to make his inheritance official. Soon, however, he sees Prince and Garfield, and attempts to chase them ending up ragged clothed and exhausted. He threatens the solicitors and Lady Westminster with a crossbow, but he is finally bit in the bottom by Jon's dog, Odie, who has arrived at the scene attempting to recover Garfield. In the end, Smithee and the Police arrive, Dargis is arrested and Liz and Jon get engaged.