After a fairly promising getaway, "Romancing the Stone" gradually chases its tail into enough melodramatic dead ends to deteriorate into an expendable runaround, all too easy to shrug off as a miscalculated clone of "Raiders of the Lost Ark."

The promising aspects cluster around the heroine, Kathleen Turner as a romance novelist named Joan Wilder whose utterly unglamorous personality is meant to provide endearing contrasts with the extravagant, libidinous tendencies that gush out of her fiction. In fact, the movie opens with an amusing fake-out, depicting the denouement of Joan's latest opus, a palpitating Western, as she types it out.

A perilous real-life adventure intrudes on Joan's literary trances, theoretically exposing this solitary dreamer to authentic dangers and then raptures in the company of a handsome mercenary (Michael Douglas). But the conception isn't clever or conscientious enough to adhere to the logic of her character profile. Though fitfully thrilling and amusing, her adventures degenerate into a muddle. Neither screenwriter Diane Thomas nor director Robert Zemeckis, good-humored as they strive to be, maintains a coherent perception of how the plot should be contrived to trump the heroine's overactive fantasy life.

The heroine responds to a life-or-death plea from her sister, who's being held hostage in Colombia. Bewildered but loyal, Joan flies to her rescue carrying some kind of treasure map coveted by the captors, bossed by Zack Norman, playing a transplanted wretch from New York who keeps pet crocodiles and exasperates his fuming cousin and errand boy, Danny De Vito. There's also a rival group of menaces, some kind of militia headed by a sleazo officer (Manuel Ojeda) active in gem smuggling.

Compared with an object like "Raiders' " Ark and antagonists like the Nazis, the set-up in "Romancing the Stone" is poorly contrived to sustain a chase thriller. The Ark, like the Maltese Falcon, remained a MacGuffin extraordinary enough to seem worth the chasing. Not so the Stone, which lacks a richly violent legend and looks awfully pasty when it ultimately surfaces.

The reptile phobia shifts from snakes to crocodiles, who are saved for climactic payoffs alternately gruesome and facetious. Turner goes to considerable effort in the early stages of the movie to prepare you for Joan's eventual, gratifying emergence as a heroine in her own right--ideally, something that surpasses the romanticized misconceptions that falsify her books.

Unfortunately, all her amusing shrinking-violet affectations end up wasted, because the character fails to blossom out of timidity. Moreover, she's imagined demonstrating physical bravery of a kind that suggests Mike Hammer crossed with Conan the Barbarian, not quite the essence that seems appropriate in Joan's case.

Perhaps the major disillusioning slip-up is that Turner and Douglas don't generate a compelling semblance of romantic chemistry and compatibility. She modulates from breathlessly shy to breathlessly ga-ga while he never quite anchors the hero with a believable core of ruggedness or resourcefulness. These two prove such an uncharismatic romantic team that in retrospect the casting seems decisively self-defeating, better equipped to thwart romantic fancies than exalt them.