"Blast From the Past" is one of those high-concept movies in which the gimmick becomes an albatross around the story's neck.

An obliging comedy about a 35-year-old man-boy raised in a backyard bomb shelter by parents who panicked during the Cuban Missile Crisis, the movie stars Brendan Fraser as the wide-eyed innocent making his first foray to the surface in 1998 on the assumption that civilization was destroyed by nuclear war.

What he finds instead is the San Fernando Valley and a romance with Alicia Silverstone.

The movie starts hilariously enough, with Christopher Walken as a commie-paranoid, Norman Rockwell kind of guy dragging his pregnant wife (Sissy Spacek) into the underground bunker -- which he's fashioned into a replica of their home -- after mistaking a plane crash in their neighborhood for nuclear first strike.

Locked away for half a lifetime, they raise their (literally) sheltered son with 1962 sensibilities, while above ground a funny subplot is taking shape in the form of a soda shop built where their house once stood, that over the years declines into a boarded-up punker bar in a bad part of town.

When Walken determines it's safe to surface, he ventures up at night and misreads the ghetto that used to be their suburban cul de sac as a post-apocalyptic world worse than he'd imagined and decides to stay below.

Fraser, playing the grown son, ventures forth in search of supplies with a fist full of money and a cigar box tucked under his arm, filled with vintage baseball cards and what he thinks are worthless stocks.

While "Blast" stays at least affable throughout, it's at this point the movie starts to slowly unravel. The fish-out-of-water gags are unusually fresh (Fraser sing Perry Como tunes and is dumbstruck by the girls in a Hawaiian Tropic ad), but the rather sloppy script is often steered into tedious and forced gimmickry in order to advance the plot.

Fraser meets the ringlet-curled and spaghetti-strapped Silverstone in a sports memorabilia shop where she works (as if!), when he cashes in a few baseball cards. She gets fired for leveling with him about what the cards are worth and reluctantly agrees to help him round up supplies even though he won't tell her why he needs, for instance, two trucks full of frozen beef.

After rebuffing his giddy, inept romantic advances, she also agrees to help him shop for a wife (still under the impression that L.A. is post-apocalyptic, he's planning to return to the bunker and procreate), leading to the predictable make-over from the Silverstone's gay roommate (Dave Foley) and her inevitable jealous epiphany that -- surprise! -- she's in love with him.

My complaints about "Blast From the Past" are mostly these nit-picky scripting snafus that can be blamed on the screenwriter trying to shoe-horn elements into the story that just don't fit. But there seem to be hundreds of these things, most of which could have easily been fixed. Example: A girl like Alicia Silverstone's unlikely employment in a baseball card shop could have been explained away if the shop were owned by her father or an uncle. How hard is that?

But a more prominent problem is Silverstone herself, who makes a minimal effort in her lifeless role as a supposedly skeptical and sassy girl of the '90s, and who has zero chemistry with Fraser. He's the one who holds the movie together with his well-timed, farcical performance.

(To be fair, the screenplay gives Silverstone very little to do. But in the hands of say, Reese Witherspoon or Christina Ricci, The Girl in this movie would have had enough spark to gloss over some of the plot holes.)

The early scenes in the bunker are actually the funniest part of the movie, which is almost worth seeing just for Walken. He gets to bust out of his tough-guy veneer and have some fun playing his uptight '60s conservative for big laughs. But once it's in the second act, the groans catch up to the laughs and ultimately that race is a photo finish.