"If You Love This Planet" is a no-nonsense lecture by Dr. Helen Caldicott about the danger of nuclear war. Produced by the National Film Board of Canada and nominated for an Academy Award as best documentary short subject, this frank, nonhysterical, craftily edited film deserves to be widely seen and discussed.

The Justice Department seems to have assured just that by declaring the 26-minute film a propaganda menace, ordering a disclaimer affixed to it and notifying the distributor--tiny Direct Cinema of Los Angeles--to submit a list of organizations that want to see it, and the names and addresses of any persons "receiving 100 copies or more."

At the moment, there are only 50 prints of the 16-mm film, which has been available for a rental fee of $40 to schools, libraries, churches and community groups exclusively through Direct Cinema.

On Tuesday, however, the Biograph in Georgetown will offer the documentary--along with two other Canadian films deemed by Justice to be propaganda--at 5, 7 and 9 p.m. for the admission price of $1. The voting members of the Motion Picture Academy will see "If You Love This Planet" at a screening March 11 in Los Angeles, and a good turnout now is assured. The Canadian Embassy aired the films in cassette form yesterday for press and TV crews, after which the cassettes were whisked off to Capitol Hill for several command performances.

"If You Love This Planet" is a classic point-of-view documentary. Its point of view is that nuclear war is bad, very bad--and possible, very possible.

It takes the form of a lecture by antinuclear activist Caldicott intercut with illustrative and ironic material from film archives.

Wearing a red dress and addressing her unseen audience in schoolteacherly tones, Caldicott begins by citing Einstein's E=mc2. Then director Terri Nash cuts to footage of the Hiroshima bomb, then to a snippet from "Jap Zero," a 1943 American war propaganda film starring Ronald Reagan and then to a wartime newsreel, narrated by Ed Herlihy, which exults that "the natural powers of the universe are harnessed in the new atomic bomb." President Truman is seen promising a "rain of destruction from the air" unless Japan surrenders.

In Hiroshima, Caldicott says, "A little boy was reaching up to catch a red dragonfly in his hand against the blue sky, and he disappeared." Cut to bomb blast; add ethereal music; return to Caldicott at her podium.

About 100,000 were killed by that bomb, she says, but today nuclear bombs are much more efficient. A 100-megaton bomb would be easy to make, and if exploded in space could wipe out an area the size of six western states. Today America has 30,000 to 35,000 nuclear weapons, enough to "kill every Russian human being 40 times."

Cut to "Jap Zero." Ronald Reagan, American combat pilot, says, "How soon do I get to shoot one down?" The answer: "Soon enough."

Nuclear-armed nations, Caldicott says, are untrustworthy. They are like "two boys arguing in the sandbox." They are prepared not simply for war, but for "extermination. What you do to cockroaches." We are being prepared psychologically, she says, not only for a nuclear war, but also for the notion that such a war might have a "winner." These are "lunatic-like" concepts.

"A bomb dropped on a nuclear reactor would contaminate an area the size of West Germany," she says, attributing the comparison to Scientific American. "If you reach a fallout shelter, you won't be able to emerge for six to eight weeks," she says, quoting the New England Journal of Medicine. "When you do come out, all the architecture, all the music--imagine a world without Handel, Beethoven or Brahms--will be gone, all the literature . . . "

Survivors of the exchange, she says, then will die from starvation, radiation, epidemics (because their immune system will be weakened by radiation), sunburn (because the atmosphere's protective ozone layer will have been destroyed), blindness and grief. "I, myself, will die of grief," she adds.

"There's lots to do," Caldicott tells her audience in Plattsburgh, N.Y., "You're next to a SAC airbase here. Close it down."

"If You Love This Planet" may sound like one of those driver education movies designed to make you sick to your stomach, but that is not its effect. Caldicott does not rant; her tone is that of a teacher addressing the PTA. The juxtaposed footage never is ghoulish, and the cuts to President Reagan as an actor are less satirical than they are poignant.

Direct Cinema's Mitch Block--who works out of his home in Los Angeles--says he now must recall his 50 prints and attach a 79-word disclaimer that concludes, "does not indicate approval of the content of this material by the United States Government." He says that will cost money, but he is more upset that, as a result of the disclaimer, "groups with any public funding may be nervous about showing it."