What Rep. Morris K. Udall (D-Ariz.) calls "Udall's law of unintended consequences" went into operation immediately upon publication of the news that the Justice Department had foiled a cinematic effort by the Canadian government to foist on us their deplorably negative views about nuclear war and acid rain.

The first thing that happened was that the National Press Club lunch at which Dr. Helen Caldicott, the star of "If You Love This Planet," which has not a single decent thing to say about nuclear weapons, was transformed into a box office hit. Before Justice struck, only 30 tickets had been sold.

The occasion gave Sen. Edward M. Kennedy (D-Mass.), who was introducing Caldicott, the welcome opportunity to make a ringing denunciation of the president's Orwellian conduct and to demand a Senate investigation.

The Canadian ambassador weighed in with both the Justice Department and the State Department, and protested that judging three of his country's documentaries to be "political propaganda" was not exactly either friendly or neighborly.

Flustered by the fuss, the head of the Justice Department's Criminal Division, D. Lowell Jensen, sent over to the Canadian Embassy last Friday for the prints and conducted a weekend review at a higher level. But these senior critics upheld the original decision.

Henceforth, organizations and individuals who choose to expose themselves to the hazards of these films will first see a disclaimer which warns that they were made by a registered foreign agent and the U.S. government does not necessarily approve of them. Those who see them may also be reported to the attorney general.

The rush is on for prints of the anti-nuclear film. Rep. Edward J. Markey (D-Mass.) has arranged for it to be shown on the House closed circuit television system. The local Biograph Theatre is having a special showing tonight. The manager does not know whether G-men will be on hand to take down names and faces.

Possibly, Justice Department film critics have inadvertently promoted "Planet's" chances for an Oscar. It has been nominated. Certainly Justice did not intend that as a consequence.

If you see "If You Love This Planet," you will understand why some red-blooded Reagan zealot reached for the branding iron.

It wasn't just that Ronald Reagan has a cameo role in it. He is shown in a clip from "Jap Zero," in a combat flyer's rig, eagerly asking, "How soon do I get a chance to knock one of them down?" It is in the gung-ho spirit of the gloating newsreel foot-age, announcing the dropping of the bomb over Hiroshima.

It wasn't the scenes of Hiroshima victims and their hideous burns and facial distortions. An American film, "The Final Epidemic," showed as much.

Nor was it Caldicott's unrelenting description of the vaporizing, decapitation, gassing and frying that accompany nuclear attack. President of the Physicians for Social Responsibility, she travels about the country saying such things by night and by day, and admits she is "emotional" about the question of exterminating the human race.

No, unbalanced as Caldicott's presentation might seem to a loyalist who is trying to support Reagan's nuclear arms buildup, it is possibly her cheeky call to action that did it.

She urges the people of Plattsburgh in her audience to close down the Strategic Air Command air base nearby.

She further tells them to take their babies to Washington and plant them on the desks of hawks.

"Say to Sen. Henry M. Jackson or Sen. Jesse Helms, "Why are you not representing the life of this baby?' "

That's pretty bad, you must admit. But there is more.

"Set your naked toddlers loose in the Senate chamber," she abjures them in her ringing Australian tones.

Plainly it is a call to sedition and riot.

But "Acid Rain, Requiem or Recovery" is another story. A more tactful, neutral, inoffensive presentation of a fearful problem that is being visited on one country (theirs) by another country (ours) cannot be imagined.

In a totally unemotional voice, the narrator recounts the death of fish and the corrosion of buildings and gravestones caused by the fallout from industrial smokestacks. The Environmental Protection Agency is mentioned only once, and favorably. A good American firm in Canada is cited for reducing its emissions by 50 percent.

The harm it can do has already been done. Thousands of garden clubs and environmentalists have seen this subversive footage.

Justice Department wizards figured out that President Reagan's principal political problems are the scandal at the EPA and the nuclear freeze movement, and reasoned from that that the thing to do was to keep quiet about them. So they have said that is is un-American to be against nuclear war and acid rain. It's an odd message at this time, but Reagan is letting it stand.