C

There are exceptions, of course, but as a general rule of thumb you can take it as a given that when a TV show, book franchise or movie sequel resorts to an “It’s a Wonderful Life” episode, it’s a clear sign that the wellspring of inspiration has run dry. That’s certainly the case with “Shrek Forever After,” which is being sold—honestly, one hopes—as the last in the venerable animated series that began so pleasantly nearly a decade ago. This fourth installment demonstrates that the big green ogre has definitely passed his prime.

Actually, that’s how Shrek (voiced by Michael Myers, who frankly seems to be losing interest, if the relatively sleepy delivery is any indication), feels after a few years of marriage to Fiona (Cameron Diaz). Their three kids are demanding imps, there are constant household chores, and his celebrity status brings a flood of starstruck tourists to see him. That makes him an easy target for the villainous Rumpelstiltskin (Walt Dohrn, the behind-the-scenes head of story, and as voice talent game if hardly extraordinary), who persuades him to sign away one measly day of his past life in return for a chance to be a true, scary ogre again and bust loose for twenty-four hours. Of course, the cunning villain erases Shrek’s birthday, which means not only that after his twenty-four hours are up he’ll cease to exist, but that he’s now in a new world in which he’s never existed, and where his old friends don’t know him.

It’s an awful world, of course, with Rumpelstiltskin having taken control of the kingdom by duping Fiona’s parents into surrendering it to him—all because Shrek hadn’t been around to rescue and marry the girl. With his army of witches he controls everything, waging war against the ogres led by Fiona, now a warrior princess. To break the spell and return the world to its old state, Shrek must make her fall in love with him and give him a true- love smooch before the day ends. And to do that he’ll need to enlist his old friends Donkey (Eddie Murphy) and Puss in Boots (Antonio Banderas), who of course have no idea who he is.

There are occasional lines in “Forever After” that recall the subversive pop-culture wit that marked the earlier installments, though in ever-decreasing percentages. But for the most part the movie feels as tired as Shrek says he feels. Even when the jokes are fairly good, the delivery is mostly as flabby as the new but definitely not improved Puss, whose girth as the pampered pet of Fiona would put Garfield to shame. (Banderas, however, extracts as many laughs as he can from the cat’s obesity without descending to the manic quality Murphy again brings to Donkey.) But too often the gags will make you gag. The household jokes in the first reel are mostly of the fart, poop and pee variety so common in children’s movies nowadays (the chore Shrek is always called back to perform involves overflow in the outhouse), and though things improve somewhat as the movie goes on, it’s not by much. The overall effect is pretty dispiriting.

As is customary with today’s animated pictures, this DreamWorks effort comes in 3-D, and unlike “How to Train Your Dragon,” it employs the format to allow for calculated in-your-face effects. Otherwise the visuals follow the template of the previous “Shrek” movies, which is beginning to look a bit old-fashioned by now, though its familiarity itself is a virtue.

“Shrek” isn’t the first series to go into premature decline, of course. But in this case it’s happened awfully quickly. Maybe cinematic ogres age in dog years.