"Washington Mistress" may not exactly blow the lid off the Capitol dome but the CBS movie, at 9 tonight on Channel 9, certainly has a handle on sex, politics and sexual politics in powertown. If most TV films are really B movies, this one is definitely a B+.

We first meet the heroine, Senate aide Maggie Porter, in front of a Senate office building for a confrontation with a man who appears to be her husband but turns out to be something less. Then a limo ride through Rock Creek Park takes her into a tunnel and a flashback to five years earlier, when she met the dashing young rat Michael Reynolds, a lobbyist who knows Washington's ropes and has a socially prominent -- thus politically advantageous -- wife.

Maggie falls in love with the guy over her own moral and practical objections; eventually he tells her, "I found us an apartment," she becomes pregnant ("If you love me, have the baby," he urges) and she waits faithfully for him to make good on the pledge to leave his wife and marry her, letting her own promising career fall sacrificially by the wayside.

Explaining the situation and the way she fell for the lobbyist to her father, Maggie says, "He seemed to be a part of what Washington is all about," which is accurate in more ways than one. Maggie's plight can symbolize the disillusionment that can crush high ideals in any city; the movie is sort of a "Ms. Smith Goes to Washington."

"Mistress" is more attuned to the rhythms and realities of the capital than any TV production since "Washington: Behind Closed Doors." But the real revelation of the movie is Lucie Arnaz in the role of Maggie. As she has already proven in stage roles, Arnaz has come a long way since the days when she played perky foil to her mother, Lucille Ball, on one of Lucy's old TV shows. Arnaz is a compelling and intelligent presence in this film; she lifts the character beyond the level of suffering soap opera heroine, and she finds unexpected shadings and subtleties in Maggie's victimization. It's a very strong, very admirable performance.

As the lobbyist, Richard Jordan also gives dimension to what might have been merely a cad. But the script by Audrey Davis Levin is unusually well thought out, and director Peter Levin, as he did with "The Marva Collins Story," demonstrates how clarity in a TV-movie need not be colorless.

The film grew out of two articles in the Style section of this newspaper by Sally Quinn (Quinn is credited as "creative consultant"), but is a fictional story devised by Audrey Levin, not an adaptation of the article. That doesn't mean however, that a lot of people won't be able to see a lot of truth in it.