There are two "Cocoons." One was directed by Ron Howard, and it has all the warmth of his comic touch, his respect for his characters, his way of plugging into the humanity of a situation. The other, a bloated special-effects extravaganza, seems to have been directed by a particularly slavish camp follower of Steven Spielberg. The two movies mix like sugar and sludge; the result is a terrific little movie ankle-chained to a gorilla.

Walter (Brian Dennehy) and Kitty (Tahnee Welch) rent an estate next to a retirement community; they seem to be vacationing cousins, but they're actually disguised aliens, former residents of Atlantis who have come to recover the ground crew they left behind. They hire a boat skippered by Jack Bonner (Steve Guttenberg), but it's not a pleasure cruise -- they set about recovering the cocoons on the ocean floor, ovoid, rocklike containers for their long-lost brethren.

The cocoons are kept in the estate's pool; when three chums from the retirement home (Wilford Brimley, Hume Cronyn and Don Ameche) sneak in for an afternoon swim, they discover that the pool has acquired the power to rejuvenate them. They collect their girlfriends and spouses (Maureen Stapleton, Jessica Tandy and Gwen Verdon) and, in what is for them as daring as the raid on Entebbe, find there's life in the old guys yet. Only Bernie Lefkowitz (Jack Gilford) demurs -- he feels they're cheating nature.

Finally, the aliens come clean, revealing the glowing, fetal beings beneath their latex guise. And that's when the movie starts to disintegrate. As long as it sticks with the oldsters, "Cocoon" has a rare kind of charm. There's a comic poignance in these lives, as they debate the relative merits of Ex-Lax and prunes, or deal with the loss of memory. When the cocoons do their work, the movie cuts loose, as these couples dance the night away in "The South's Finest Climate-Controlled Ballroom," carrying on with a verve they haven't known in years. There are few treats equal to watching Ameche, splendid in a white suit, crooning "Some Enchanted Evening," or Brimley sneaking up on his wife in the shower and saying, "Want a piece of candy, little girl?," few smiles that match Cronyn's killer grin.

Even these segments, though, have their problems. The way the movie emphasizes the revival of sexual potency nearly to the exclusion of everything else starts to grate -- certainly, other aspects of rejuvenation would be just as important (the ability to play sports again, or to read), but "Cocoon" makes little of them. There's a vengefulness here that you don't expect from the good-natured Howard -- when the characters go astray in "Cocoon," the story swiftly punishes them. And screenwriter Tom Benedek has slighted the women characters; it's a shame to waste talents like Stapleton and Tandy, but they're not given anything to do.

There would be more room for the women if Benedek and Howard weren't so busy working in extraneous aspects for the youth market. The romance between Welch and Guttenberg, for example, is just a third wheel; the boat skipper's difficulties in falling for an alien only distract from the core of the story. The lines are clever enough, but these scenes barely hint at emotions Howard was able to capture in the love between man and mermaid in "Splash." Most of the problem stems from the players. Welch is lovely, to be sure, but so plastic that you're not surprised to discover her earthly self is just a costume. The boat skipper's role needs someone who can be touched, in some way, by the aliens -- someone like the Henry Winkler of Howard's "Night Shift," or even Howard himself -- but Guttenberg is so spectacularly self-involved he drains the feeling from every scene he's in. The aliens mean nothing to him because he's always in his own world, and a puny one at that.

What's left is a mishmash of references to every Spielberg movie from "Jaws" to "E.T.," including a drawn-out finale that apes "Close Encounters" so closely you feel like you're reading yesterday's newspapers. Howard's great strength as a director is that he's able to tap into emotions that are real and familiar -- even when his characters are thrown into fantastic situations. What makes the best scenes of "Cocoon" so vivid, like a tearjerker in which Brimley, fishing with his grandson, says goodbye in one long take, are their sense of lived-in life; the trumped-up special-effects pseudo-religiosity of the movie's end is outside his me'tier. It's sad to watch such original gifts wasted on someone else's movie. Cocoon, opening today at area theaters, is rated PG, and contains some profanity and sexual themes.