Former U.S. ambassador to Chile Nathaniel Davis and two ex-embassy aides filed a $150 million libel suit yesterday against the makers of a controversial book and film they said portray them as ordering the death of an American free-lance writer during the 1973 Chilean military coup.

The complaint, filed in federal court, said statements in the film, "Missing," starring Jack Lemmon and Sissy Spacek, and the book on which it was based, "The Execution of Charles Horman: An American Sacrifice," are "false, unfair, inaccurate and defamatory."

The 30-year-old Horman disappeared days after the coup that overthrew Chilean President Salvador Allende Gossens, allegedly because he had stumbled on covert American assistance to the coup plotters. "Missing" details a character, based on Horman, who is slain by the new junta, allegedly with U.S. Embassy involvement or approval, in order to silence him.

Allegations of complicity by the embassy and of a later cover-up designed to prevent Horman's father from learning the truth about his son's death have been hotly denied by Davis and the two men who joined in filing the suit, former U.S. consul Frederick D. Purdy and retired Navy Capt. Ray E. Davis, former head of the U.S. Military Group in Santiago.

"I feel it's a very bad thing -- and bad for the United States government, the Foreign Service and the military service -- to have it open season on making allegations of awful malfeasance," Nathaniel Davis said yesterday in a telephone interview from Newport, R.I. "The people's confidence in their government has suffered a certain amount of buffeting as a result of this book and movie."

Named as defendants were "Missing" director Constantin Costa-Gavras; Universal City Studios Inc. and its corporate parent, MCA Inc.; New York lawyer and author Thomas Hauser; Harcourt Brace Jovanovich Inc., publishers of the hard-cover book, and the Hearst Corp., whose Avon Books division published the paperback version under the title "Missing."

"I can't say the lawsuit comes as a complete surprise," said Universal attorney Sheldon Mittleman. "We stand by the picture. We don't believe it's defamatory and we will defend it vigorously."

"Missing" begins with a declaration that some names in the film have been changed "to protect the innocent and also to protect the film" but adds that the film is based on a true story with "incidents and facts documented."

"That's almost the opposite of a disclaimer," Mittleman said. "The film is an accurate description of events, with legitimate public comment. We're entitled to fair comment on U.S. government activities in South America."

That view was echoed yesterday by Horman's father, Edmund Horman, a New York City industrial designer who met with Nathaniel Davis in Chile during attempts to learn of his son's fate.

"The movie has accomplished a great deal of what we hoped to accomplish," Horman said in a telephone interview. "It's good, as far as it goes. There's a lot more that could be said. But the movie has impact, it's accurate."

Horman is not named as a party in the complaint filed yesterday and declined to comment on it.

The State Department defended Nathaniel Davis early last year at the time "Missing" was released, calling department efforts to locate Horman "intensive and comprehensive."

Investigators found "no evidence of any involvement by any United States government personnel in the disappearance and death of Charles Horman," an official statement said.

A body with fingerprints matching Horman's was discovered about five weeks after he disappeared from the Santiago home he occupied with his wife, Joyce.

Horman's widow quoted neighbors as saying her husband was led from the house in the company of several men in civilian clothes.

Davis' complaint, filed in U.S. District Court in Alexandria, specified numerous bits of film dialogue it said were designed to show the plaintiffs "ordered or approved the order for the murder of Charles Horman."

"I have reason to believe that my son was killed by the military," the elder Horman, portrayed by Lemmon, says in the movie.

"Where did you hear that?" inquires Ray Tower, the U.S. military aide allegedly representing Navy Capt. Davis.

"I do not think that they the Chileans would dare do a thing like that unless an American official cosigned a kill order," Horman responds.

In another scene, the elder Horman confronts the U.S. ambassador:

"What is your role here besides endorsing a regime that murders thousands of human beings?" Horman demands.

Ambassador: "Let's level with each other, sir. . . This mission is pledged to protect American interests, our interests, Mr. Horman."

Horman: "They're not mine."

Ambassador: "There are over 3,000 U.S. firms doing business down here and those are American interests. In other words, your interests. I am concerned with the preservation of a way of life."

"To put that in the mouth of a United States ambassador is pretty appalling," Nathaniel Davis said yesterday. "I know very well I never said those words. It doesn't represent anything I believe."