Most children’s franchises throw out quality in exchange for fleeting amusement, but you’d think DreamWorks would take quality further into consideration for such a high-grossing series as the “Shrek” movies. With the fourth installment, the studio has dropped the ball. “Shrek Forever After” is a fun and enjoyable 90 minutes, but beyond that it feels more like a direct-to-video sequel than the culmination of a successful animated film series.

The first “Shrek” came out nine years ago, and it was a fresh, welcome sensation. It reached critical acclaim and quickly became DreamWorks Animation’s most bankable creation. The story of an unusual – meaning large, green and nonhuman – “prince charming” (not to be confused with the actual Prince Charming, a bratty character in the series) is a valuable tale for children and adults alike, who are all too often swayed by 20th century, pretty-faced Disney animation, where the hero always looks like Prince Charming himself.

In this fourth theatrical outing, Shrek (Mike Myers) has settled down with wife Fiona (Cameron Diaz) and triplets Farkle, Fergus and Felicia into their cozy swamp house. He’s content to pass the days as a family man, receiving daily visits from best friend Donkey (Eddie Murphy) and nodding to the passing tour bus that points him out as a public attraction. Shrek becomes wary of the routine though, and uncomfortable with his new softie reputation.

Staying rooted in fairy tales, “Shrek Forever After” brings in a classic character to mix things up — Rumpelstiltskin (Walt Dohrn). He tricks Shrek into making a deal: In exchange for one day of his life, he gets to live one more day as a true ogre, free to roam and terrorize humans at will. The jealous Rumpelstiltskin chooses the most inopportune day — Shrek’s birthday — and takes it away. And thus commences the “Back to the Future” existential paradox. How does Shrek get back to his life if he was never even born?

On a critical level, “Shrek Forever After” is like a typical unnecessary sequel. It returns Shrek to his old locations and threatens to render all his previous efforts pointless. It represents the creators’ attempt to squeeze out one more film without bothering to progress the story. The film is a big character arc, and not much else. Considering the richness of the personalities and the odd charm of their world, it’s unfortunate that more effort wasn’t given to developing the story as a whole.

All is not lost though. On an entertainment level, “Shrek Forever After” has at least a bit of the original “Shrek” charisma, even after losing some of its magic. The movie has the same beautiful animation style that made each of its predecessors a popular hit. In true DreamWorks fashion, the film is delightfully bright, shiny and realistic in its own disfigured way. The RealD 3-D print capitalizes on what is quickly becoming an animation standard. It’s rather well done — that is to say, not especially gimmicky.

There are also a few brilliant moments scattered throughout the film, which, not surprisingly, mostly involve the Murphy-voiced Donkey. Even in the most uninspiring moments, his character is adorably irreverent, and it’s a marvel that his shtick still feels fresh.

Overall, the film isn’t much of an addition to the series, save for the opportunity to see the loveable set of characters one last time. On a narrative stage, the “Shrek” series leaves a lot of its potential unrealized and unexplored. There was clear room in the franchise for a fourth film, but with its lack of development, “Shrek Forever After” is a franchise ending letdown.