"Hero at Large," an unheralded starring vehicle for John Ritter, turns out to be a lightweight pleasure. A fresh variation of vintage romantic comedy conventions, it may emerge as one of the surprise hits of the year once kids and family audiences discover its cheerful appeal.

Ritter, astutely avoiding the smirky cuteness that constantly threatens to wear out his welcome on "Three's Company," proves an ideal, winning comic lead. He plays an earnest, aspiring young actor, Steve Nichols, who is drawn into real-life feats of heroism as a consequence of casual employment as one of 62 stand-ins for Captain Avenger, the hero of a low-budget imitation of "Superman."

"Captain Avenger" (the original title of the film) open a saturation engagement in New York. Steve is one of the actor hired to dress up in the superhero's costume and make promotional appearances at participating theaters.

Returning to his Greenwich Village apartment while still in costume, Steve makes a quick stop at the corner grocery just before closing time. Moments later two young punks invade the premises and attempting to hold up the mom-and-pop proprietors -- the Rothenbergs, adorably impersonated by Henrietta Jacobson and Micheal Gorrin.

Unable to resist his decent impulses or the opportunity to play Captain Avenger for real, Steve Springs into action and rousts the bad guys. The Rothenbergs offer effusive thanks and a carton of milk on the house. Steve accepts humbly and fades into the night, his true identity still a mystery to the grateful grocers.

The next day the Rothenbergs have gone public with their story. Press interests run high. Steve himself is interviewed by a TV newscaster (Jane Hallaran) at one of the theaters. Although he conceals his role in the previous night's adventure, Steve is inspired by the surge of good will he's suddenly created. He tries to express his contentment without giving himself away, and soon finds himself trying to sustain that impulsive flight of heroism. The double life complicates his attempts to make headway with the girl across the hall and entangles him in a majoralty campaign.

Every so often a young director aspries to make a new romantic comedy in the tradition of the Frank Capra standards like "Mr. Deeds Goes to Town," "Mr. Smith Goes to Washington" and "It's a Wonderful Life." As a rule, the aspiration leads only to the minor disgrace of an ineffective, embarrassing anachronism.

But the youngish director of "Hero at Large," Martin Davidson, and the veteran screenwriter, A. J. Carothers, who used to contribute regularly to Disney comedies, have made a movie that actually recalls the spirit of Capra at his most skillful and endearing while creating a distinctive new comic protagonist in a persuasively modern frame of reference.

Davidson, Carothers and Ritter contrive to reaffirm the square virtues embodies by Steve Nichols -- friendliness, sincereity, honesty -- without looking or sounding pathetically square in the process. Davidson's last feature, the high-school comedy "Almost Summer," never made it to Washington-area theaters, but it revealed a promising flair for comedy pacing and character interplay when it surfaced on Home Box Office. "Hero at Large" confirms and enhances that promise.It's as bright and proficient as the last comedy last produced by MGM (and also photographed by David Walsh), "The Goodbye Girl."

Carothers' script has a couple of lulls, but they're mercifully brief. A more serious deficiency is the lack of an inspired kicker at the fadeout to complete and summarize the happy mood audiences are likely to take away from this film. Carothers' dialogue is remarkably good: funny and clever without degenerating into glibspeak, and rarely out of character. For example, there's a delightful exchange in which Ritter calls up his agent, played by Allan Rich, only to get his records voice, which growls, "Want to know if you've got a job? The answer is no." After a moment's reflection, Ritter answers, "This is Robert Redford. I'm seeking new representation. Sorry you were out."

Bert Convy distinguishes himself in the expertly written role of a master public-relations con artist. He's such an accomplished hydrocrite that you can't help but admire his smug self-confidence. Convy puts an ironic shine on utterances like "Somebody has to care" that seems brilliantly disarming. You know he's lying through his teeth, yet what a dazzling performance!

The texture is constantly enlivened by throwaway show business jokes and a parade of amusing supporting players. As the neighbor Ritter falls for and successfully courts, Anne Archer seems a slightly incongruous element. She and Ritter establish a pleasant rapport, but the chemistry of screen romance isn't quite there.

Archer is a lanky, generous-featured actress who may have the unique drawback of possessing too much facial distinction for her own good. Her large eyes and mouth are photogenically commanding, but they may be grander features than most lines or leading men can compete with. Ritter's soft, round facial contours might harmonize better with similar feminine contours.

But this is quibbling of a highly esoteric kind. "Hero at Large" is first and foremost an infectiously good-humored entertainment.