Alfred Hitchcock once summed up the difference between surprise and suspense perfectly. Imagine two people sitting at a table talking. Suddenly, a bomb explodes under the table. The audience, unaware of the bomb, is surprised.

Now, let's say the same two people sit and talk at the same table, but in this instance, the audience knows the bomb is under the table and will explode in 15 minutes. The characters chat away, oblivious, while the audience longs to warn the characters.

As Hitchcock, the Master of Suspense, pointed out, "In the first case we have given the public 15 seconds of surprise at the moment of the explosion. In the second we have provided them with 15 minutes of suspense."

Hitchcock's explanation is now famous, both because he used the ticking time bomb device so effectively and because his description of it is so accurate.

If used deftly, the ticking time bomb device never gets old and can be inserted into endless contexts in myriad forms. It worked in Hitchcock's day, and the movie "In Time" proves that it works just as well today.

"In Time" is overall a mediocre movie lacking any surprises, other than surprisingly nonsensical plot developments. But the basic concept is interesting and resonates in America circa 2011.

Will Salas (Justin Timberlake) lives in a near future in which the human life span is controlled genetically. Each human being is designed so that on everybody's 25th birthday, a glowing green clock implanted on their left arm begins to count down their final 365 days. At the end of that year, they die.

You can earn extra time, however, by working, trading, stealing, etc. Time is the new currency, as the movie says, and has completely replaced money.

For the wealthy, all of whom live in one district of what was once the U.S., this system means immortality. They can go on living forever, never aging, as long as they keep their time accounts full.

For the poor, like Salas, this system means living in constant fear of dying. Will and his mother, Rachel, (Olivia Wilde) never have more than a few days left on their clocks, so they must work, borrow and scheme constantly to stay alive.

This scenario is interesting but hardly original.

The society pictured in the film, with its complete segregation between rich and poor and its leisure class living in paradise while the poor live in squalor and fear, originates in Fritz Lang's 1927 science fiction masterpiece "Metropolis" and has been recycled in numerous movies.

The notion that everyone in a society must die at a relatively young age and that some people run to escape this fate, like Will is going to, comes directly from the 1976 classic "Logan's Run."

"In Time" melds those sources with a revenge story.

Rachel dies early on because Will meets her a mere second late. He has been given a century of time by a wealthy man who wishes to die. Yet he can't reach her in time to share his new wealth.

So Will buys his way into the wealthy district, falls in love with a malcontent rich girl named Sylvia (Amanda Seyfried), and the pair become a sci fi Bonnie and Clyde, hoping to crush the system by robbing time banks.

The performances are all fine. Cillian Murphy turns in a humorously hard-nosed role as a Minuteman, a cop who hunts down time thieves.

The story, however, becomes quite silly, playing out in one absurd, illogical action sequence after another.

Despite that, "In Time" is fun to watch, mostly because of that bomb under the table.

The movie saves itself each time those little clocks get near zero. It's a testament to the power of a ticking clock and to one of the cinema's oldest tricks.

Jeff Marker is a media studies professor at Gainesville State College.