On his first jag as "The Greatest American Hero," William Katt takes three giant steps, wobbles into the air, careens off a brick building and flits splat into a billboard. The police arrive and toss him and his red tights -- and his cape -- directly into the loony bin.

From there, there's no way to go but up, up and away. "Hero," a two-hour movie (at 8 tonight on Channel 7) preparing the way for a new weekly ABC series , might have been just another superhero spoof if not for the deft, wry touch of Stephen J. Cannell, of "Rockford Files" and "Tenspeed and Bown Shoe" fame. There's something engaging and brisk about the distinctive spin Cannell puts on a ball.

Katt, as boyishly ingenuous as ever in his pre-Raphaelite (or are they Raphaelite?) curls, plays a high-school special-education teacher who, during a field trip in the desert, has a visitation from a glowing UFO. It doles out a supersuit to him, along with a set of instructions which he naturally loses. Then he goes forth to eke a little justice out of everyday life, a hopeless task to be sure.

Mainly this consists of thwarting a diabolical scheme to take over the government of the United States by force or violence. And other such sundry little chores.

The film's cuteness quotient does rise to threatening levels now and then, but basically this is a trim, chipper, breezly funny movie. It opens, as TV movies almost never do, with a clever vsual stunner: A spider's silent progress up a desert slope is suddenly interrupted by the roar and zoom of dune buggies. They're pursuing, it turns out, an FBI agent who will later materialize from the saucer.

The casting is in every instance fortuitous. D.D. Spradlin does right-wing heavy bit better than almost anyone. Connie Sellecca makes her ingenue-attorney part something to be reckoned with and savored; her badinage with Katt when the heat's on recalls the snappy patter of Carrie Fisher and Harrison Ford in "Star Wars'" episodes four and five.

Robert Culp, as another FBI agent, supplies a very amusingly deadpan center around which the action can revolve. ("Fortunately," he says, people who claimed to have seen Katt fly "got written off as whacko cuckoos.") And there's a major discovery and potential teen-age heartthrob in Michael Pare as a sophisticated Fonz called Tony Villicana. This kid will be heard from.

The superhero-as-klutz shtick gets tiresome after awhile, but Katt's essential freshness sees most of it through. He first tries on his suit in the privacy of his own bedroom while his young son watches Superman cartoons in the next room. Later he is forced to change in a public men's room, interrupted by a father and son who assume this is some new variety of L.A. weirdo. "Oh, hi," Katt says innocently when caught mid-tights.

Spradlin's villain is a sinister millionaire who wants to abduct the president and replace him with the simpering vice president he owns. His hired thugs are toughies with shaved heads and white T-shirts (at least one of them, it is cleverly revealed, is a female) who spout Jesus-freak rhetoric at their victims -- a touch that, as they say, may offend some viewers, though probably not to the point of firing off angry letters by the thousands.

Cannell wrote and executive-produced the film, which benefits from his laconic-acebic dialogue. After ridiculing agent Culp in a diner for his "Elliott Ness" attire, Pare is reminded that the man carries a gun. "But then, like they say, clothes don't make the man," he ruefully backs off. Katt apologizes for him, and when this is met with a rebuff, he snaps, "I said I sorry. What do you want -- a trip to Europe?"

And he later complains to Sellecca that, when airborne, things are not as easy as they look: "I navigate like I was hit with a can of Raid." The look on his face when he perceives the extent of his superpowers is a wonderful Everymannish wow.

In fact, "The Greatest American Hero" is pretty wowable all around.