''THE LOST HONOR OF KATHRYN BECK'' is a cautionary tale - earnest, gripping and intense. Mostly, its target is police detention and intimidation of witnesses. At the same time, it has it in for the press. Politically, it is trendy and up to date, with something in it for everyone, although in its heart it is a melodrama. The two-hour movie will be seen on CBS at 9 tonight.

The movie stars Marlo Thomas and Kris Kristofferson. Miss Thomas is Kathryn Beck, the hard-working owner of a small catering business. Mr. Kristofferson is Ben Cole, a suspected bank robber and Weatherman terrorist. There is a resonance here of the 1981 armored-car robbery in Rockland County in which a Brink's guard and two police officers were murdered. Two of the radicals found guilty were once Weathermen.

Mr. Kristofferson meets Miss Thomas at a party. They spend the night in her apartment, but he leaves before dawn. The police who pursue him burst into the apartment, humiliate and intimidate Miss Thomas, and then hold her as a witness. She, of course, knows nothing about Mr. Kristofferson's criminality, and neither, for that matter, does the viewer. We have only the word of the police about his past, and the police are suspect; we see how they abuse Miss Thomas.

On the other hand, Mr. Kristofferson does keep a pistol on the front seat of his car, and he is hiding out. The movie has it both ways here. Mr. Kristofferson is an attractive actor, with a taciturn charm. It's hard to believe he's guilty of anything. On the other hand, why is he hiding out, and why are the police so intent on arresting him, anyway?

Meanwhile, a newspaper editor begins to harass Miss Thomas. ''Terrorist's Lover,'' one of his headlines says; ''Uncontrollable Sex,'' says another. Actually, he seems more representative of Fleet Street than of anything in this country. In pursuit of the story, he slips into the hospital room where Miss Thomas's mother lies terminally ill. The shock of his visit kills her.

The malefactors, obviously, are the police and the press, and not Mr. Kristofferson. After he has escaped, leaving Miss Thomas in the lurch, he calls her. ''You're like a beautiful boy when you sleep,'' she says. She also says she doesn't know what he's supposed to have done. Presumably, she hasn't read the stories beneath the headlines. Anyway, Miss Thomas is not the slightest bit angry with Mr. Kristofferson, although in the penultimate scene she does shoot the newspaper editor. Viewers will cheer here. It's easy to see why the Administration kept reporters away from Grenada; you wouldn't have a man like that around, either. Thus the film, which is handsomely designed by Peter Larkin, gives us messages from the left and right: the police and the press should both be curbed. ''The Lost Honor of Kathryn Beck'' is based on the novel ''The Lost Honor of Katharina Blum'' by Heinrich Boll. Mr. Boll argued that all Germany was in decay. The film does the same, only using another country.
By JOHN CORRY