Life is full of disappointments. From Michael Dukakis’s presidential campaign to WOW! Chips (the ones with Olestra and all those uh … unusual side effects), things that seem too good to be true generally are. But few disappointments can begin to approach the horror of watching a failed big-screen adaptation of a favorite childhood cartoon.

“TMNT,” director Kevin Munroe’s attempt to revive the wildly popular Teenage Mutant Ninja Turtles movie franchise after a 14-year hiatus, is certainly more entertaining than a Dukakis campaign speech. Unfortunately, much of the charm and appeal of the cartoon that won legions of followers during its run in the late ’80s and early ’90s is lost in its new computer-generated incarnation.

Born not out of the beloved series of our youth, or the previous three live action films, “TMNT” takes its cues from the new “Teenage Mutant Ninja Turtles” cartoon (extant since 2003). Perhaps it’s the influence of anime and new animation styles, or perhaps it’s just the gradual deterioration of our society, but whatever the cause, “TMNT” just seems to lack the originality and character that really made the earlier Ninja Turtles so cowabunga-worthy.

The film opens with a very cursory and surprisingly listless introduction to the Turtles as they leap from rooftop to rooftop, then spends the next 20 minutes or so on some of the most convoluted, absurd and flat-out tedious exposition ever to curse a kids’ film.

Instead of explaining the Turtles’ history (as the past films did), “TMNT” delves into a tale of prehistoric Latin-American warlords, inter-dimensional portals and monsters from outer space. It seems that, in days of yore, some great warrior in search of immortality opened this portal to another dimension. Unfortunately, in addition to immortality, the portal brought a curse, turning his army to stone and unleashing 13 beasts on mankind. Here’s the real kicker: the only way to reverse the curse is to re-open the portal, which is only possible every 3000 years, and return the beasts.

Now, you probably guessed it, 3000 years have passed, and the warlord, now known as the fabulously wealthy businessman Maximillian Winters (voiced by Patrick Stewart), must try to open the portal. “What does this have to do with our ‘Heroes in a Half-Shell’?” you may ask. Who knows, and indeed, many viewers (especially those who usually populate a PG film audience) may lack the patience to find out.

Meanwhile, back at the ranch, there’s trouble in the sewers. Shredder — the greatest villain to grace the world of Saturday morning cartoons — has apparently been defeated, and the Turtles are having familial difficulties. Master Splinter, the lovable rodent Mister Miyagi, has sent Leonardo off to Latin America to work on his leadership skills. The rest of the Turtles have gotten day jobs and stopped fighting crime. Even though Raphael is secretly still doing the vigilante thing, there’s something distinctly depressing about watching Donatello work as an IT support operator.

After being contacted by April O’Neil (Sarah Michelle Gellar), no longer the beleaguered reporter we knew and loved but a Lara Croft-style archeologist, Leonardo returns to New York — a undifferentiated clone of Metropolis or Gotham. From here on out, the film alternates between the Turtles’ domestic disputes and their battles with monsters, Winters’ stone soldiers and the Foot Clan (Shredder’s minions who now work for Winters).

Unlike the other superhero franchise films (“Spiderman,” “Batman,” etc.), “TMNT” gets bogged down with its villains. The real comic book’s villains are as much a part of the narrative as the heroes, smoothly fitting in to make the fantasy world complete. Here, however, the monsters don’t really feel like they belong; they are intruders who lack their predecessors’ snappy interactions with the Turtles — interactions that had allowed a certain amount of character development.

As a pure action/adventure cartoon, the film is serviceable. The fight sequences are, at times, both exciting and entertaining, and some of the monsters are fairly impressive and wouldn’t look out of place in “The Lord of the Rings” or some other CGI creature feature. Unfortunately, if you’ve come for the Turtles, you’re in for a big disappointment.

Michaelangelo and Donatello barely appear on screen, and when they do, they are merely caricatures without depth and wit. In fact, “TMNT” shows none of the easy humor and warmth that made the old Turtles’ banter work. In CGI, they all look more like reptiles and less like humans, and, as we watch one pointless action sequence after another, we can’t help but feel as though “TMNT,” like its protagonists, is ultimately cold-blooded.