A scientist in 1899, Hartdegen endures a tragedy that makes him obsessive about the prospect of altering time. (One character later describes him as “a man haunted by those two most terrible words: What if?”) So he uses his quantum-mechanical skills to build a device capable of transporting him back to that fateful moment. Unfortunately, his dabbling there present a conundrum: If he can return to the past, why can’t he change it? Only the future holds the answer.

Perhaps filmmaker Simon Wells might one day be seeking the same solution. The great-grandson of “The Time Machine” author H.G. Wells was given a genetic opportunity of a lifetime, and he squanders it on a mildly entertaining movie that utterly collapses during the final act.

This isn’t like the recent “Planet of the Apes”-style epilogue that offered a tacked-on annoyance to an otherwise passable experience. This is a monstrously misguided climax that is neither exciting, emotionally involving nor well-staged.

The movie provides the sensation of watching two entirely different tales that have been spliced together. Maybe it has something to do with the fact that Wells (“The Prince of Egypt”) succumbed to exhaustion with three weeks of filming left, and hack director Gore Verbinski (“The Mexican”) was brought in to handle much of the underground scenes involving the cannibalistic community of 800,000 A.D.

For as many ideas that Wells and screenwriter John Logan come up with to improve on his ancestor’s fascinating novel, something else is inserted into the story that detracts. The hero is wisely given real motivation for tackling time beyond just scientific curiosity. But there’s no urgency to Hartdegen’s stay in the Morlock/Eloi future, which finds the human race divided into two distinct species. In the book and the original 1960 film version, the Morlocks’ theft of the time machine forces the scientist to enter their lair to retrieve it.

In the update, he can pretty much leave any time, and given the circumstances, it’s hard to rationalize why he’d want to stay.

Ultimately, he turns into a stock action-hero no more interesting than the one played by Rod Taylor in the fondly remembered first flick. There’s no question that Pearce is a superior actor to the laconic Taylor who is coincidentally a fellow Australian. But it’s also obvious that Simon Wells is not as skilled a director as George Pal (“The War of the Worlds”), whose handling of Oscar-winning special effects in the initial version was equally effective without the advent of digital technology.

A few actors help take up some slack in the personality department. Comedian Orlando Jones (“Evolution”) is well-employed as a holographic library guide from the near future, whose all-pervasive knowledge can’t quite disguise his disdain for frivolous questions. Jeremy Irons is slightly more menacing as the bad guy (credited as the Uber-Morlock) than he was in a similar role in 2000’s “Dungeons &

Dragons.” He at least doesn’t have to shout as much, instead projecting a cold menace while buried under layers of pale makeup and long white hair which makes him look like a possessed Edgar Winter.

However, Irish pop singer Samantha Mumba barely registers as one of the comely Eloi who takes a fancy to Hartdegen. While she needs to represent someone for whom he would abandon his chance of returning to the past, she instead just strolls around flashing a beatific grin and speaking in ambiguities.

The movie does boast one first-rate action sequence in the lattermost version of the future. Here, Hartdegen must fight a group of hunter Morlocks as they track their human food source during a scene that begins suspiciously like one from either version of “Planet of the Apes.” It’s fast-paced and scary, as the hulking, simian beasts seem incapable of being defeated. But like the rest of “The Time Machine,” it offers a lot of promise before ending abruptly in a noisy, unsatisfying finish.

Maybe next time …