Dermot Mulroney is the first to admit that his show biz saga, brief though it is, reads like a Hollywood cliche'.

It's the one about the good-looking 22-year-old -- in this case born and raised in Alexandria -- who decided about midway through college that what he really wanted to do was act. As luck would have it, a William Morris agent visited his college during his senior year. The scenario that followed went like this:

The agent produced a contract. "She said, 'Here, sign this.' So I did," recalls the 5-foot-10, 150-pound Mulroney.

The agent then asked him to relocate. "She said, 'Come to Hollywood.' So I did," Mulroney continues.

Auditions followed. And more auditions. For three whole months. (Bear in mind that in The Land of Broken Dreams, there are performers who have auditioned and struggled for decades in anonymity.)

Mulroney, who was being supported by his parents at the time, was on the verge of applying for a waiter's job. Then came the call to read for "Sin of Innocence."

He never got around to memorizing menus and prices. Instead he memorized lines.

His Big Break airs tonight at 9 on Channel 9 -- a CBS made-for-TV movie. Starring Bill Bixby and Dee Wallace Stone, "Sin of Innocence" also stars Mulroney and Megan Follows as a teen-age stepbrother and stepsister who decide to play house.

Mulroney sums up the plot this way: "We fall in love and get caught in bed and it's a real mess for everyone concerned."

Currently at work on an episode of the syndicated series "Fame" ("Uh, this role is definitely a challenge. For starters, I play a dancer, so I wear tights. I also wear a dance belt -- which is something no human being anywhere should have to do"), Mulroney admits that so far it's been pretty easy.

"It sounds like I sort of fell into all this, right?"

But he's quick to add, "I know I'm extraordinarily lucky. I'm also pretty realistic about all this."

How real is Mulroney?

"I'm realistic enough to know that it's just possible I may never work again, ever. Then again," he adds, "I'm at a marketable age with a marketable look and all that sort of thing. So chances are I will work again. The question is in what medium? And what will the quality of the TV shows or films be? One would like it to be the next Coppola film. But of course, you don't always get what you want in this business."

His father, Michael Mulroney, is with the Washington specialty tax firm of Lee, Toomey & Kent. His mother, Ellen Mulroney, is "a mother and an amateur actress active in regional and community theater," according to her son. He adds that she's just wrapped a local production of "Agnes of God."

It was at Northwestern University in Evanston, Ill., that Mulroney decided to try acting. (He has a BS degree in radio-TV-film.) He appeared in several Northwestern stage productions, including the annual "The Mee-Ow Show" (an improvisational revue) and that old standard, "The Mikado." "I don't sing, exactly. I sort of shout in tune," he says.

It was William Morris agent Barbara Gale who signed Mulroney to a contract. When he came west, the only professional experience he could claim was a public-service announcement for a Chicago high school that encouraged kids to remain in school rather than drop out, and an industrial film that was "an anti-teen-alcoholism thing."

Now he lives in the Palms district of West Los Angeles and shares an apartment with two former Northwestern classmates. "Most of my friends are involved in some way with the industry. If they're not acting, they're writing. Or working behind the scenes. We're all kind of in the same place right now, trying to make it. It's sort of happening for us."

When "Sin of Innocence" first happened for Mulroney (who was the first to be cast), he spent an entire day reading opposite "just about every girl in L.A., I think," he laughs. "So I had no complaints about going to work that day."

Once the cameras began rolling in January, in Fort Lauderdale and Key Biscayne, Fla., he found "the situation more than the character to be a stretch." The reason? "The scenes are emotionally draining, the situation is complicated."

He admits that "Sin of Innocence" is, in a sense, his calling card. Whether it will net further jobs will depend somewhat on ratings as well as reviews. (TV Guide's Judith Crist called Mulroney and Follows "impressive young players . . . very good indeed.")

Mulroney also agrees that the latest screen trends can affect his career. So for reasons of practicality, he says, "I'm only half sorry to see the 'Porky' type movie disappearing. Because I have to be realistic -- it's in movies like those that I could get work.

"If the next genre of films is about 40-year-old women, I'm in big trouble. Well, unless they happen to have a son."

Asked if his realistic attitude stems from having heard a stream of hard-luck stories (which are continually echoing throughout the industry), he says, "It's not that I've heard the hard-luck stories. It's just that I pretty much know how it goes. You can't count on anything -- if you do, you'll be depressed. So you have to just work and hope it will happen.

"You have to knock on every piece of wood you see."