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So, this is a live-action adaptation of the animated series Scooby-Doo, Where Are you? (or any of the various Scooby-Doo cartoons). The movie does a fair amount of lampooning various aspects of the cartoons. And I thought most of the cast did a pretty good job of portraying the characters from the show, especially Matthew Lillard as Shaggy. Scooby himself works pretty well in CGI form, as well. I also really liked that Daphne was played by Sarah Michelle Gellar (from Buffy the Vampire Slayer, which is clever casting, since Buffy and her friends had always called themselves "the Scooby Gang," a pop culture reference to the cartoon). And Velma was well-played by Linda Cardellini. Freddie Prinze, Jr., wasn't really bad as Fred, just kind of "meh." I mean, he was good enough at playing the role as written in the movie, I'm just not sure how much he was really like the character from the cartoon.
Anyway... it starts with the Mystery, Inc. gang wrapping up one of their typical mysteries, unmasking a supposed ghost. But immediately after the case is resolved, the team breaks up, because Velma is tired of Fred always taking all the credit for solving mysteries, when it's her plans that actually deserve most of the credit; and because Daphne is tired of always getting caught and needing to be rescued. Fred also decides to quit the group, apparently so as not to be left out of the whole "quitting" thing, I guess. The only ones who don't really want to quit are Shaggy and Scooby (which is a bit ironic, since they're the ones who are the most scared of ghosts and monsters and whatnot, in spite of the fact that none of that stuff ever turns out to be real).
Anyway, the film then flashes forward to two years later. Everyone gets letters from someone named Emile Mondavarious (Rowan Atkinson), the owner of a theme park called Spooky Island, to which he invites them. The team is a bit reluctant to work together again, but they do so anyway. Apparently, the college students who come to Spooky Island for Spring break start behaving oddly, by the time they leave, and Mondavarious believes someone's cast a spell on them. So, the members of Mystery, Inc. begin their investigations (separately, at first, aside from the virtually inseparable pair of best buds, Shaggy and Scooby). But eventually they end up working together. Oh, also, on the plane ride to the island, Shaggy met a woman named Mary Jane (Isla Fisher), with whom he has a lot in common. So she becomes a potential love interest, and later gets involved with all the weird stuff on the island. (Velma also meets a guy on the island, but we don't learn his name, and he's of no real importance to the story.)
I don't want to say anything about what's going on on the island, or who's behind it all, but it does seem to involve some genuine supernatural elements, unlike any of their old cases. Nevertheless, it ultimately is sort of like all their old cases. Anyway... I'd understand if a lot of critics don't care for the movie, or even if a lot of regular viewers don't care for it. But personally I found most of it amusing, and more entertaining than I generally found the original cartoon (even if much of my appreciation of the movie stems from how surprisingly similar the movie is to the source material). And I liked the special effects, and whatnot. The music was decent. And I dunno what else to say. It's not what I'd call a quality movie, but I did think it was fun. (Oh, I should also mention that I think I saw this movie like on TV once, or something, before I got it on DVD. And there was a scene I think I saw that time that wasn't in the movie on the DVD... but the DVD had some deleted scenes that were fun to watch, and one of them may have been the scene I was thinking of. I don't remember exactly.)
There's also a sequel, Scooby-Doo 2: Monsters Unleashed.