Capricorn One—the first manned mission to Mars—is on the launch pad. The bewildered crew of Brubaker (James Brolin), Willis (Sam Waterston), and Walker (O. J. Simpson) are removed from Capricorn One and flown to an abandoned desert base. The launch proceeds on schedule, and the public is unaware that the spacecraft is empty. At the base, the astronauts are informed by NASA official Kelloway (Hal Holbrook) that a faulty life-support system would have killed the astronauts during the flight, and they must help counterfeit the television footage during the flight to and from Mars. Initially they refuse, but Kelloway threatens their families if they do not cooperate.
The astronauts remain in captivity during the flight and are filmed landing on Mars within a studio located at the base. The conspiracy is known to only a few officials, until alert technician Elliot Whitter (Robert Walden) notices that ground control receives the crew's television transmissions before the spacecraft telemetry arrives. Whitter reports his concerns to his supervisors, including Kelloway, but is told that it is due a faulty workstation. He mysteriously disappears before he can finish sharing his concerns with journalist friend Robert Caulfield (Elliott Gould). Caulfield discovers that all evidence of his friend's life has been erased and begins investigating the mission, surviving several attempts on his life in the process.
Upon returning to Earth, the empty spacecraft burns up due to a faulty heat shield during re-entry. The "astronauts" on the spacecraft are revealed on live television to have died due to the heat shield. The captive astronauts board a plane to be placed in the spacecraft, but the plane unexpectedly turns around and returns to the airfield. They realize that something has gone wrong with the re-entry process and that officials can never release them because doing so would automatically expose the hoax. They break out of their confinement and escape in a plane, which runs out of fuel soon after take-off. Forced to crash-land and stranded in the desert, they attempt to return to civilization while being pursued by two helicopters. Realizing that the chances of exposing the truth is increased if they split up, they start walking in three directions. Kelloway send helicopters after them; Willis and Walker are found and killed, while Brubaker is the only one to avoid capture.
Caulfield's investigation leads him to question Brubaker's "widow" after reviewing a conversation between the astronauts and their spouses aired on live television where Brubaker mentioned a previous family vacation that Mrs. Brubaker seemed confused by. She says that the vacation Brubaker mentioned never took place, and that the family had gone to another location where a movie was being filmed, and that Brubaker had told his wife that with enough special effects and movie magic, anything could be made to look real. As he investigates further, he is arrested by federal agents when cocaine is planted in his home, and later fired from his job.
Another reporter tells Caulfield about the military base and he goes there, finding it abandoned. Looking around, he finds a necklace given to Brubaker by his wife and concludes that the astronauts were in the hangar. With the help of crop-dusting pilot Albain (Telly Savalas), he searches the desert and rescues Brubaker from the men in the helicopters. The helicopters chase their plane but are destroyed after being blinded with crop spray.
Ultimately, Caulfield and Brubaker arrive at the astronauts' memorial service, exposing the conspiracy.Elliott Gould as Robert Caulfield
James Brolin as Colonel Charles Brubaker, USAF
Sam Waterston as Lieutenant Colonel Peter Willis, USAF
O. J. Simpson as Commander John Walker, USN
Hal Holbrook as Dr. James Kelloway
Brenda Vaccaro as Kay Brubaker
Karen Black as Judy Drinkwater
David Doyle as Walter Loughlin
Robert Walden as Elliot Whitter
Telly Savalas as Albain
David Huddleston as Congressman Hollis Peaker
Lee Bryant as Sharon Willis
Denise Nicholas as Betty Walker
James Sikking as Control Room Man (as Jim Sikking)
The film was originally scheduled to debut in the United States in February 1978, but good preview screenings and delays in Superman caused it to move to June. Capricorn One became the year's most-successful independent film.
Hyams later said:
Audiences just stood up and cheered at one point in the film. It wasn't because it was such a great movie, it's just that certain movies strike certain chords with people. In a successful movie, the audience, almost before they see it, know they're going to like it. I remember standing in the back of the theater and crying because I knew that something had changed in my life. Sitting on the film cans outside the screening room, I felt my cheeks were wet with tears. A bright man, [studio executive] David Picker came over to me and said, 'You're going to have a lot of new best friends tomorrow. You better know how to handle it.'
AllMovie critic Donald Guarisco wrote: "This agreeable high-concept effort is one of Peter Hyams' most accomplished films. The script's conspiracy-theory premise requires a major suspension of disbelief, but Hyams makes it worthwhile for those willing to make that leap."
Conversely, Vincent Canby of The New York Times called the film "an expensive stylistically bankrupt suspense melodrama" while describing much of its screenplay as "humorless comic-strip stuff."
The film received a 61% positive "Fresh" rating on the review aggregator Rotten Tomatoes.
The film is recognized by American Film Institute in these lists:2001: AFI's 100 Years...100 Thrills – Nominated
Two novelizations of the film were written and published by separate authors. The first was written by Ken Follett (under the pseudonym Bernard L. Ross) and published in the United Kingdom; the other was written by Ron Goulart and published in the United States.
The Follett novel is notable for giving Robert Caulfield more development than the movie does, including giving him something of a relationship with CBS reporter Judy Drinkwater (who has more time in the book than in the movie) and ending the book with him and Judy. The story saves his career and results in his being employed by CBS.
Clips from the faked Mars landing scenes have been used for illustration purposes in various moon landing hoax conspiracy documentaries, notably the Fox TV show Conspiracy Theory: Did We Land On The Moon and Bart Sibrel's film A Funny Thing Happened on the Way to the Moon (2001). The latter also features a still shot from the hoax scene on the DVD's front cover.