As the momentous anniversary of their whirlwind romance draws near, Jon Arbuckle gets ready to create a memorable surprise for his devoted veterinarian girlfriend, Doctor Elizabeth "Liz" Wilson, by visiting her in London without prior announcement. Little does Liz suspect that Jon has a thoughtful plan to pop the question during her business trip. Joining him in this covert operation are his playful pets, Garfield and Odie, who sneak into Jon's luggage as if they were furry little ninjas. Once they arrive at their hotel, the two mischievous cats become restless and escape their room, setting off to explore the charming streets of London.
At the same time, a curious will-reading occurs in the picturesque English countryside at Carlyle Castle. The solicitors, Mr. Hobbs, Mr. Greene, and Mrs. Whitney, gather to conclude the estate matters of the late Lady Eleanor Carlyle. To everyone's astonishment, she bequeaths her grand estate to her beloved feline companion, Prince XII, who looks remarkably similar to Garfield. This surprising inheritance infuriates Lady Eleanor's greedy nephew, Lord Manfred Dargis, who stands to gain the estate if Prince were to pass away. Seizing his chance, Dargis captures Prince and throws him into the river.
Coincidentally, Jon discovers Prince escaping from a drain, all the while mistaking him for Garfield. Unbeknownst to Jon, Prince’s loyal butler, Smithee, is desperately searching for the real Garfield, who is roaming the streets of London. In a comical turn of events, Smithee mistakenly scoops Garfield up and brings him to the Carlyle estate.
Within the lavish estate, Garfield now enjoys an extravagant lifestyle, supported by Prince's farm animal entourage, led by the noble bulldog majordomo, Winston. These animals express such respect for Garfield, who becomes their protector against the dastardly Dargis, intent on demolishing the estate and its inhabitants to build a resort. Dargis fears losing ownership of the property once he believes Prince has returned, and he repeatedly attempts to eliminate Garfield, only to be thwarted by the clever animals, who have grown fond of their new companion.
As Garfield and Prince settle into their new roles, they start to build a beautiful friendship—Garfield teaches the animals to whip up delicious lasagna, while Prince learns the joys of pet life. However, as time passes, they both long for their previous lives, especially Garfield, who feels the sting of nostalgia when he overhears the animals bemoaning his lazy and self-absorbed nature.
In a twist of fate, Garfield and Prince encounter each other again, both wishing to reclaim their former existences. Driven by empathy for Prince and his animal friends, Garfield rallies them to unite against Dargis. Their fates intertwine further when Jon and Odie unexpectedly stumble upon the unfolding drama at the estate, coinciding perfectly with Liz’s visit.
When Dargis’s sinister plans come to light, Garfield and Prince mock their nemesis, who threatens the solicitors at gunpoint while also holding Liz as a hostage. In a moment of heroism, Jon brandishes his crossbow, but Dargis warns him that he would kill Jon if he tries to interfere.
Finally, Garfield and Prince team up to defeat Dargis, aided by Smithee's timely notification to the authorities, which leads to Dargis's arrest. Reflecting on his transformative journey, Garfield transitions from being a mere rival to facilitating Jon's heartfelt proposal to Liz, which she graciously accepts.
