The system. You can't beat it.

At the beginning of the trial that climaxes "The Ambush Murders," on Channel 9 tonight at 9, a picket in front of the courthouse carries a sign reading "Down With the System."

The movie, based on the nonfiction book of the same name by Ben Bradlee Jr., is the story of the system taking four years to meander through to justice. Since it's a morality play, the outcome is never in doubt. The idea is to raise the possibility that the system doesn't/can't/won't work. It's a dim possibility (I mean, think about it) but a sure-fire plot device.

It opens with two cops responding to a burglary call, and getting shotgunned to death in an ambush in a black section of Lindero, Calif., in 1971. Circumstantial evidence points to Ray Ellsworth, a black housepainter and activist played by Dorian Harewood. So do a variety of witnesses -- a convicted perjurer, a known jailhouse informant, a transsexual, a flaky girl who says she can't tell when she's telling the truth or not, and a cop who's on record as hating "niggers."

After two mistrials, Ellsworth gets a new lawyer, Paul Marshall. "I was hoping for a flamboyant movement lawyer," he says. "What do I get? Plain white bread." Marshall, played by James Brolin, keeps telling him that "the system works." The system is not to be confused with the district attorney giving orders from his golf cart. Or with what Ellsworth describes as "liberals who rally around the more popular cause of freeing the chic radical celebrity, Angela Davis." (Imagine being an actor having to say a line like that!) Or with Ellsworth himself, described by his lawyer as "self-righteous, hostile and arrogant."

Everything proceeds in a murky and evocative texture of ambiguities -- lots of dim lighting, squad car lights bursting out of the darkness, grimy evil generally rampant, and a lot of nice acting turns. Did they have to include a black guy in the jailhouse singing "Midnight Special" with the white guy accompanying him on the guitar? Did they have to make one of the detectives so sodden-facedly villainous? Well, probably yes, that being the television movie system. Despite the scattering of cliche's, it works.

As does The System when the lawyer interrogates the key witness, a cop, and proves by scientific deduction of the "there-was-no-moon-on-the-night-of-June-5th" variety that he's wrong. Science and reason prevail, along with the noble calm of the lawyer who takes the case even though it will hurt his career, of course. Not to mention a kindly judge who seems to embody the meaning of it all when he assures the nervous transsexual, "You can trust me." Interestingly enough, he/she doesn't, but viewers will.