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``Cocoon'' begins with a boy gazing through a telescope at the night sky. Moments later an eerie glow shines from the heavens, and then we're in the depths of the ocean, swimming past the ruins of a strange undersea kingdom. It's the standard opening for a Steven Spielberg-type fantasy, with the child on the brink of some rousing adventure. So it's quite a surprise when that little boy turns out to be a minor character in ``Cocoon'' -- playing second fiddle to his grandparents.

Are the movies ready for this?

Can a yarn about senior citizens -- in a Florida retirement home, no less -- compete with ``The Goonies'' and the usual summer glut of teen-market trivialities?

Nobody knows for sure, but it's refreshing to see a young director like Ron Howard and a major studio like Twentieth Century-Fox give it a try.

The heroes of ``Cocoon'' are three old men, living out their days in continual boredom. For them, a big adventure means sneaking into their neighbor's house for a swim in the pool. One day their bodies and minds start changing, though -- they feel young again, their energy revives, their sex drives perk up. That pool has become a fountain of youth, and the secret lies in some odd-looking objects nestled at the bottom.

They're cocoons, of course, left on Earth by outer-space visitors many centuries ago. An expedition has returned to bring them home, and the ordinary swimming pool has been turned into an ``energized'' storage depot. The aliens don't like the idea of three rejuvenated codgers splashing around their precious cargo. But the codgers love the idea of growing young again, and want to share the bonanza with their families and friends.

Much of the way, watching ``Cocoon'' is like seeing two different movies. One plot deals with the three men and their newfound youth. The other focuses on the aliens as they hunt for cocoons in the deep-sea ruins of Atlantis, helped by a bumbling boat captain. While these stories are connected by the cocoons, they don't really mingle until the old folks meet the space folks head on, which happens quite far into the film. Until then, we viewers bounce happily between two separate fantasies. This makes for a bumpy trip, but a nicely unpredictable one.

``Cocoon'' has no bad guys. While this weakens the movie's tension and suspense, it lends a friendly atmosphere that suits the summer season, as does the sunny Florida setting.

The picture is something more than a lightheaded romance, however. Questions about the meaning and value of old age are broached, if not explored. Issues of illness and death also make an appearance, embodied most touchingly by an old couple who choose not to join the fountain-of-youth gang.

Although these matters aren't treated deeply, their presence gives extra resonance to the film -- which chucks them out in plenty of time for the wishful finale, a Spielbergian extravaganza in every sense.

Ron Howard was a natural choice to direct ``Cocoon,'' since his last picture was ``Splash,'' another fantasy in a watery vein. An outgoing filmmaker with a good sense of comic timing, he makes the most of both ``Cocoon'' story lines and integrates the large cast into a smooth ensemble.

The performers range from young Steve Guttenberg, surprisingly solid as the boat captain, to a roster of Hollywood veterans: Hume Cronyn and Jessica Tandy play one couple; Wilford Brimley and Maureen Stapleton are another; Don Ameche and Gwen Verdon a third. In the middle, generationally speaking, is Brian Dennehy in a superb portrayal of the chief extraterrestrial, just about the nicest fellow in any movie this year.

The rating of ``Cocoon'' is PG, reflecting a good deal of sexual banter as the elderly heroes start recouping their energies.