Change must come to all things, and the Aardman folks have bowed to the inevitable: Their new feature, “Flushed Away,” is a computer-generated extravaganza.

The good news is, the Aardman humor is intact, and so are the google-eyed character designs. The film is a busy, cheeky crowd-pleaser … and yet, some of the old charm is gone.

“Flushed Away” is about rats. Who doesn’t love rats? Don’t all step forward at once. All right, so the sewer-dwelling rodents are not the cuddliest creatures, but they have problems too.

Most notably, an elegant rat named Roddy St. James (voiced by Hugh Jackman) finds himself uprooted from his posh life as a London pet when he is rudely flushed down the toilet by an intruder.

Below the streets, Roddy discovers a vast ratopia, where he must aid a tough sewer-boat owner, Rita (Kate Winslet), against the designs of a large, green, power-mad toad (Ian McKellen).

At first the toad wants a precious ruby, then abruptly shifts his attention to destroying rodent life during the World Cup (the story feels as though different screenwriters changed course mid-movie).

McKellen’s grand, warty toad is the movie’s best character, and his French cousin, Le Frog (Jean Reno), is a worthy relative. Also good, and bearing the brunt of the movie’s copious slapstick pratfalls, are two rat henchmen voiced by Bill Nighy and Andy Serkis. There’s also a group of squealing slugs who become this movie’s equivalent of the penguins from “Madagascar.”

It moves along at a rat-a-tat (pardon the phrase) pace, with verbal jokes as quick as the sight gags. The voices are expertly done, and the heroes are good role models. Well, except for being rats.

The preview audience laughed a lot. I chuckled, and smiled, but somehow the bigness of “Flushed Away” left me unconvinced. The tenderness and quirks of the early Aardman is gone, and many of the jokes here struck me as surprisingly coarse, even if the idea is to attract the grown-up audience along with the kids. This is a neatly executed movie, but bring back the clay.

When Roddy (center) is flushed away into the bustling metropolis below London, he’ll need the help of Rita (center right) if he’s going to escape the clutches of Toad (right), Le Frog, Whitey (left below), and Spike (left above) in “Flushed Away.”