Sometimes a movie gets made, and you know the people working with it are just going to have fun with the whole exercise. It's the sort of film you look at and say "this isn't going to get me any kind of awards, it's going to die in the box office, and it's going to be derivative crap, let's just ENJOY getting this paycheck."
This is one of those movies.
And it's hysterical.
It's not the kind of hysterical that happens if you show up and expect Shakespeare, it's the kind of hysterical you get if you see this movie for what it is: Bill Murray, Tim Curry and Billy Connolly just CHEWING up scenery and deciding to meta-comedy mock the complete crap they've been given to make.
To give you an idea of the level of comedy we're talking about here, the Big Bad is Connolly who is trying to steal some valued real estate so he can tear it all down and build condos. As part of his nefarious deeds he trains his pit bull Rommel to attack anything that smells like Garfield (who is posing as the rightful owner, some pampered royal cat). When the animal arrives to attack Garfield to pieces, another dog suggests he'd much rather eat the trousers that Connolly's character likes to wear. Resulting in a lot of "dog bites man in dick, man theatrically reacts" sight gags. As a result Connolly has to sell a gag in which he stands there in a very formal suit, and a 16th century groin protector that looks like a Klingon Washboard. He pulls it casually off and hands it to an orderly "polish that will you" even though the dog he's wearing it as protection against IS STILL STANDING THERE. So what does Connolly do? Play it as straight as possible but somehow mock the whole proceedings.
Likewise, when the various animals come to the aid of Garfield's defense, a movie deus ex machina used again here when Connolly's character comes under fire from geese, ducks, ferrets, and so forth, some of whom rig up complicated Rube Goldberg weaponry that provides non-lethal but hysterical results - Connolly is arrested, wild-eyed, going "It was the animals, you know" making the line as ridiculous as it was when written.
There's literally not much you need to know. Some blandly handsome fellow plays Jon Arbuckle and Jennifer Love Hewitt took time off talking about her Swarovski Crystal'd vulva to put in a forgettable job as his girlfriend Liz. They're completely dispensible and almost 100% irrelevant to the story, existing only to take Garfield to the UK for some reason. There's a subplot involving them somehow but most of it is Connolly vs various animals trying to keep up the pretense that Garfield is some royal cat, because if he exists, then Connolly can't have the property signed over to him.
But again, this is truly irrelevant. The set pieces are in there, the rest is things like Billy Connolly running around like John Cleese meta-mocking him by scampering around carrying a morning star in the vain attempt to brain Garfield, while the three lawyers brought by to sign the papers are ushered into a room while Connolly runs around getting more and more injured as time goes on. It's Tim Curry doing his finest Stewie Griffin-esque Rex Harrison snarker bit, and Bill Murray contractually fulfilling himself doing the voice of a cat making nonsense noises as his CGI animated avatar shakes his anus and upraised tail at Connolly.
It's the kind of movie that takes itself so unseriously that its credits routine is to have the two cats, Curry's Prince and Murray's Garfield doing an extensive breakdance routine to some rap music they dredged up from a D-lister to Madea up the joint. You can practically see the writing staff going "yeah at this point an American film would have some animated mouse breakdancing" and the people working off that script were like "yup, I can read the sarcasm dripping off the page, and will continue it on".
And it's hysterical. Please do yourself the favor of catching it. In the right frame of mind, it'll have you in stitches.