There is an opening sequence that's familiar from a lot of cop movies. A madman is waving a gun and threatening bystanders. He is surrounded by cops. Then the department's hot shot drives up, sizes up the situation, intuits how to push the madman's buttons, walks up to him unarmed, distracts him, disarms him, and the crisis is over.

That's how Eddie Murphy's ``Metro'' opened, and only three months later that's Wesley Snipes'' big opening scene in ``Murder at 1600.'' This cliche serves useful purposes: (1) it opens the movie with an action scene; (2) it establishes the hero's credentials as a guy who doesn't play by the rules, and (3) it's the setup for a payoff later in the movie_usually in a minor comic key.

That's what happens this time. Turns out the guy waving the gun is a depressed government commissioner. Turns out Snipes has had problems with the same agency: It wants to tear down his apartment building and put up a parking lot. In a movie involving a White House murder, the odds are excellent that sooner or later the cop and the Ppresident are going to be chatting about that parking lot.

Snipes plays Harlan Regis, a Washington police detective who is assigned when a woman is found murdered in a the White House. He gets into a territorial struggle with Spikings (Daniel Benzali), the head of the Secret Service detail, who gives him absolutely no cooperation.

``She was killed in my city,'' Regis barks.

``She was killed in my house,'' Spikings growls.

Why was a city cop assigned in the first place? ``They got you in to make it look real,'' Regis is told, and indeed there seems to be a cover-up under way.

Among the suspects: The president's son, who was a lover of the dead woman, and perhaps the president himself (Ronny Cox). A voice of reason seems to be the Nnational Ssecurity adviser (Alan Alda), who's a contrast to the demented Mr. Clean image of Spikings.

Because this is a cop movie, Regis is supplied with a partner (Dennis Miller). Miller serves no function except to take phone calls, but soon we understand: He's a seat-warmer, brought in to provide a virtual partner until the movie can supply a real one. Regis quickly meets Nina Chance (Diane Lane), a member of the Secret Service detail and an Olympic Ggold Mmedal sharpshooter. Even though they're on opposite sides of the jurisdictional divide, they share the conviction that an innocent man is being framed.

A lot of ``Murder at 1600'' is well -done. Characters are introduced vividly,; there's a sense of realism in the White House scenes, and some of the dialogue by Wayne Beach and David Hodgin hits a nice ironic note. (In a briefing, White House spokesmen are told: ``We would like you to avoid two words when talking to the media. The words are `woman,' and `murder.''') The cop and the sharpshooter achieve an easy rapport; she likes it that his apartment is filled with incredibly elaborate reconstructions of Civil War battles.

But then the movie kicks into auto - pilot. The last third of the film is a ready-made action movie plug-in. Without giving away a single secret, I can tell you that Regis and Chance find it necessary to break into the White House. And to do this, they must traverse a forgotten series of tunnels that lead by labyrinthine twists into the White House basement. The movie does what too many thrillers do: It establishes an interesting premise, and then instead of following it, substitutes standard action cliches. Will there be water, rats, electricity, dangerous secrets, hazards, security traps, flames, explosions and gunshots in the tunnel? If you think not, you haven't seen ``The Rock'' or all the other movies that inspire this sequence.

While our heroes sharpshooter are wading through the dangerous subterranean waters, let's step back and think. They need to tell the president something. He is walled off by a conspiracy. How can they get the information to him? I can think of two answers: (1) The president's son has a personal motive for wanting his father to get the information, and has complete access to him. (2) The cop is surrounded by TV cameras every time he steps outside. He could simply blurt out the truth, since there is no need to keep it secret.

Neither of these alternatives would be as much fun as breaking into the White House, but they would have a better chance of success.

The fact is, the entire movie is fiction, and so if it's entertaining me, then I'm grateful. It's only when a movie stops working that I ask questions. For example, in a later scene, Regis masquerades as a janitor and pushes a cart through the White House while holding his head down and whistling tunelessly.

Doesn't he know that holding your head down and whistling tunelessly is what *all* suspicious characters do when they disguise themselves as janitors? Isn't that like wearing a neon sign saying ``Impostor?'' I'd love to see a taut, competent police procedural based on a murder in the White House_one that followed standard procedures to see how they were warped by presidential power. ``Murder at 1600'' seems to have started in that direction, before the fatal decision was made to cut out large chunks of the story in order to import weary thriller cliches. If I want to see a movie about slogging through flooded tunnels, I'll watch ``The Third Man.''