NOTE: This spoiler was sent in by Brentage5000who says... "Before I spoil this, I just want to remind you all that Garfield?s birthday is June 18, so go buy him some lasagna right now"
The movie starts with some shots of Garfield's house that show pictures of his owner Jon with Garfield when he was just a kitten and as he (Garfield) was growing up and out. We then see Garfield (Bill Murray) sleeping in Jon's bedroom in his own little kitty bed with his stuffed bear Pooky. An alarm next to his bed goes off. Garfield hits the alarm and grumbles, "I hate Mondays." He then wakes up Jon (Breckin Meyer) by jumping on him, flushes the toilet so Jon will get out of the shower and fix his breakfast, and then trades his liver-flavored cat food breakfast for Jon's tuna fish. This is pretty much the typical routine for these two bachelors.
Later that day, Garfield is sitting next to Jon's computer when a mouse scampers out from beneath the desk. Jon yells at Garfield to get it, but Garfield just sits there, which prompts Jon to get a broom and chase it out of the house. Once it's out, Garfield goes out and talks to the mouse, who's actually a friend of Garfield's named Louis (Nick Cannon). They talk about how Louis can't be in the house when Jon's home (Louis blames it on those delicious macadamia nuts), but than Jon comes out and sees Garfield eating Louis. Jon goes back in and Louis gets spit out. Garfield goes wandering, confuses a Doberman named Luca (Brad Garret), and later runs into Nermal (David Eigenberg). Nermal tries to talk to him, but Garfield is basically a grump. At one point Nermal sees the milkman coming and wants to go get some, but Garfield has them play an astronaut game instead. Nermal climbs into a bucket while Garfield pulls a string, making the bucket go up, which makes something get hit that makes the milk bottle deposited on the step fly up to the roof where it rolls into the gutter, breaks open, and pours down into Garfield?s mouth.
Later, Garfield is watching a TV show starring Happy Chapman (Stephen Tobolowsky) that advertises his favorite cat food. The show ends, and we are treated to a brief scene between Happy and his assistant where we see Happy as an angry guy who is allergic to the cat he partners with, Persnikkity (Alan Cumming), and is envious of his newscaster brother. Back with Garfield, we see him prancing around the feet of Jon, who has just returned form the grocery with four packages of lasagna, which Garfield has eaten before Jon can come back with the rest of the groceries. Garfield is lying there moaning in ecstasy and Jon decides that he needs to go to the vet, an old high school classmate of Jon's named Liz Wilson (Jennifer Love Hewitt). As they're going, Garfield states that Jon should just tell her how he feels since he?s been in love with her since the day they met.
At the vet, Liz tells Jon that Garfield is fine except for the need for a flea dip, which Garfield treats as though it were a thing at a spa. While he gets dipped and pretends to be a celebrity in front of all the caged cats and dogs, Liz asks if she can talk to Jon in private. They go to her office and she starts talking about how nice he is and wants to ask him something, and he says that he's felt the same way for years and he is ready to have a dog, which is what Liz was going for. The dog's name is Odie and Jon, still wanting to impress Liz and kinda taken by the dog after all, promptly agrees to take it. Liz follows Jon out to the car and while Garfield freaks out about Odie (who is in his seat, no less), Liz invites Jon to a dog show she's judging the next day. He blusteringly accepts and drives off with Garfield, who is still freaking out and shouting anti-dog messages.
Once home, Nermal asks about why Odie's there but Garfield just asserts his manliness (catliness?) and goes inside to find Jon giving Odie a guided tour. That night, Garfield goes to bed and sees Odie lying on Jon's bed. He tells Odie that Jon won't let Odie sleep with him, but guess what; he does anyway. The next day, Garfield goes to his chair and starts to watch TV (specifically, the "Hey, Mama" music video) when he looks down and sees Odie staring up at him. He tells Odie to go away, watches the tube, starts bouncing to it, and finds Odie back at the bottom of the chair. Odie leaps up and starts licking him, but Garfield just pushes him down a few times before getting on the floor. They then proceed to dance with each other right out the front door, where they are observed by Garfield's girlfriend Arlene (Debra Messing) and Nermal. Garfield makes up some excuse just as Liz pulls up. Jon comes out all dressed up and gets in Liz's pickup with Odie. Garfield, panicked about being left behind, breaks his "never leave the cul-de-sac" rule and chases after them, climbing into the bed and hitting the rear window unnoticed when Liz slams on the brakes at a stoplight.
At the dog show, Liz takes Jon to the main circle and goes to the judges table while Garfield wanders off on his own, ending up right next to the performing dogs. They all chase him, causing massive chaos, and while they're gone, the song they were supposed to perform to "Hey Mama" comes on and Odie starts dancing to it. Garfield throws the dogs off his track just as Odie gets the blue ribbon from Happy Chapman, one of the other judges. Happy asks Jon if he'd consider throwing Odie in show business, but Jon refuses, although he does take Happy's card. Everybody goes home, and Garfield sneaks back inside while Liz makes a date for dinner Sunday with Jon. Garfield hits a ball across Jon's office that ultimately makes everything in it come crashing down and gets him kicked out for the night. He does some cat-erwauling on the fence while watching Odie and Jon go to sleep together. Eventually, Garfield settles in on the front porch when Odie comes outside to keep him company. Garfield turns around and locks him outside, though, to the disapproval of Arlene and Nermal, because as they say (and as Odie does) dogs run away. Odie chases after a car that outruns him. He waits at a stoplight until another car comes along and chases that, and eventually finds his way to some old lady's house. She takes him in and takes care of him, posting some lost pet posters the next day. Eventually, someone sees one of the posters and comes to claim Odie and that someone is Happy Chapman.
Meanwhile, Jon is frantically trying to find Odie. Garfield, who is feeling just a little guilty, is watching TV when Happy Chapman comes on with his new pal, Odie! He says that he and Odie are doing their last show here and that they will be leaving that night for New York City. Garfield tries to get Jon to come and see, but by the time Jon comes over it's gone to a Wendy's commercial. At that moment, the doorbell rings, and it's Liz, arriving for their date. After she confesses to having her own crush on him in high school (which does NOTHING to calm him down, I'm sure), he says that he forgot about their date. He goes to the kitchen to grab his jacket and then comes back and tells her that Odie ran away. They go out hunting while Garfield looks at the back of his cat food box and "finds" the location of the studio based on the crude and childish drawing on the back. After a couple false starts and a bagful of Nacho Cheese Goldfish, Garfield gets going on his own hunt, eventually hitching a bus into the city.
Once in the city, Garfield hopelessly wanders around complaining about how this doesn't look like the back of his box and asking anyone if they know where the studio is ("It's next to the big orange tree?" he ventures hopefully). He gets cornered at one point by some rats, but is saved by Louis, who happens to be in with the rats. Louis takes Garfield to the studio and walks him through crossing the street (which takes about half an hour). Once that's done, Louis warns Garfield to watch out for Animal Control and wishes him good luck. Garfield rejects the idea of going in the front door and goes up the ventilation shaft instead. After getting blown about by people who thought he was a blockage in the system, he works his way up to Chapman's office. He finds Odie and apologizes, but before he can break Odie out, Chapman comes in and straps an electroshock collar onto Odie that makes him do flips and stuff. Garfield is shocked and bolts after Chapman.
While this is going on, Liz and John (who by now have realized that Garfield is missing too) have found one of the signs posted about Odie being found. They go to see the lady but she says that Chapman already claimed him which causes them to head for the studio at the same time that Chapman is heading for the train station. Garfield grabs a food tray and start grinding down the banisters but someone opens a door at the wrong time which makes him fall out a window and into a truck filled with lasagna. After indulging himself, he gets out and heads for Chapman and Odie but is captured by Animal Control.
At the pound, garfield is thrown into a cage where he tells his tale to all the captured animals, a group which also includes Persnikkity, the cat whom Odie replaced (side note: Persnikkity says his real name is "Sir Roland," which is what I'll call him for the rest of the spoiler). At that point a little girl comes in with her parents looking for a kitty and Roland, Garfield, and three other cats are put in the box. Roland gets chosen, and he makes a fuss that ends with every captured animal breaking loose, including garfield, who once again heads for the station. Once there, he finds the right train...just as it's pulling out of the station. While Jon and Liz, who have just arrived, try to get the girl at the customer service to stop that train, Garfield fools around with the track controls for a few minutes, almost causing a twelve car collision, but eventually fixing it and getting the train with Odie on it back in the station. garfield gets on, finds Odie, but just as they leave they find Chapman blocking their exit. At the last minute, Roland and the other animals Garfield has run into show up and set upon him -- dogs biting, cats clawing, and rats putting the electric collar around his neck. garfield and Odie take turns shocking him until Jon and Liz show up and Jon knocks him out with one punch (granted, that probably doesn't mean much considering how beat up he already was, but still...).
A few days later, we're back at Garfield's house once more. We see Jon and Liz sitting on the couch sharing a bag of Goldfish while Garfield sits on the chair with Odie as they all watch some old romantic movie from the forties or fifties or sixties. Garfield says that he and Odie are good now because they both have one thing in common -- they both love Garfield.