Scooby-Doo, TV's comically cowardly Great Dane, is 40 years old this year. In all that time, through succeeding series on different networks, he has rarely been off the air. It is really quite remarkable, when you consider the generally poor animation, wan jokes and endlessly repeated plot line — phony spooks mask criminal enterprise undone by meddling kids and their dog — although perhaps that is also its appeal.

You know where you are with "Scooby-Doo."

If Scooby is not quite the cultural powerhouse he was back when a van with flowers painted on its side was a groovy thing and a boy might wear an ascot to high school without fear of harm, he is perhaps something more now: an icon with history, living in the collective mind of the generations who have grown up on him.

Now, a third film about the lovable dog and his human companions is showing on Cartoon Network, which is reportedly setting up yet another animated series. Scooby has already starred in two live-action theatrical films.

"Scooby-Doo! The Mystery Begins," written by Daniel and Steven Altiere (from Nickelodeon's "Gym Teacher: The Movie") and directed by Brian Levant (from the two "Flintstones" movies), is a first-rate family film, and an excellent, faithful take on "Scooby-Doo" rounded out by a surprising amount of human feeling.

It is an "origin" story, telling how Scooby met Shaggy, and Velma, Daphne and Fred, back at Coolsville High. (You will have to discount the small-fry series "A Pup Named Scooby-Doo," and should.) Someone is trying to scare the students away from the school, and anyone who has seen even three episodes of the old series should be able to work out the last 15 minutes from the first 15. Nevertheless, the film is diverting enough that you may be lulled into overlooking the obvious and into playing along with the game.

Scooby-Doo himself is rendered in CGI, less elaborately than in the theatrical films. And while there is something disquieting at times about his semi-dimensional rendering, he is arguably better animated here than in the hand-drawn original, whose signature moves — like running in place in the air, to the sound of percussion — he replicates, even as he moves and acts more often like an actual dog.

The humans, too, are less expensive than their big-screen counterparts, who included Freddie Prinze Jr., Sarah Michelle Gellar and Linda Cardellini, but they have been well cast, and make characters out of what have long been characteristics.

Nick Palatas presents an especially sweet Shaggy, woolly headed and perpetually hungry, as in the cartoon, yet also a socially awkward lonely boy grateful for friends. But Hayley Kiyoko, Kate Melton and Robbie Amell, as brainy Velma, resourceful Daphne and square-shouldered Fred, respectively, do splendid work as well.

Purists will note that Fred's hair is not blond and that Velma is Asian. Otherwise, it is business as usual: Shaggy says "Zoinks!" and Velma says "Jinkeys!"

Scooby says, "Rikes!" I didn't catch him saying, "Ruh-roh!" but he must have, right? (He is played by Frank Welker, the original and continuing voice of the cartoon Fred, and the voice of Scooby-Doo since 2002.)

And the unmasked villain grouses, as he must, that he would have gotten away with it if not for those meddling kids and their dog.