In Greece, the wife , Ellen Bracken (Susannah York), and children of American industrialist businessman Jonas Bracken (Robert Culp) are kidnapped by a radical group, the World Activist Revolutionary Army, who demand a ransom of $5 million for their safe return. Bracken raises the ransom money from selling off parts of his business empire, but the kidnappers make further demands, requiring Bracken to purchase arms and ammunition for them.

Inspector Nikolidis (Charles Aznavour) of the Greek police is put in charge of case. Jim McCabe (James Coburn), a smuggler who is Ellen Bracken's ex-husband and father to their son, reads about the kidnapping in the newspapers and meets with Bracken.

Police trace a radio signal used by the kidnappers and close in on the location, only to discover it is a decoy. A booby-trap detonates, killing several officers including Nikolidis's nephew. Nikolidis and McCabe agree that the kidnappers must be stopped, perhaps by any means. The kidnappers send a photograph of Ellen and the children as proof that they are still alive and, without the police knowing, McCabe uses a contact to trace their location based on a painted fresco in the background of the photo. He discovers that they are being held in a remote cliff-top monastery.

McCabe finds a hang glider flying circus and hires them to take part in a rescue mission. When Nikolidis discovers that McCabe has gone to free Bracken's family, the police decide to launch their own rescue plan and move in. McCabe's team use their hang gliders to infiltrate the monastery and free the hostages, but are discovered as they are leaving. While a gun battle ensues between the kidnappers and the police at the monastery, McCabe's team and the hostages are pursued and eventually escape on their hang gliders. The head kidnapper chases them in a helicopter, which McCabe forces to crash land. The head kidnapper then commits suicide rather than be captured and Bracken is reunited with his family.

A 1976 American action film (a/k/a "Assault on the Forbidden Fortress") directed by Douglas Hickox, produced by Sandy Howard, starring James Coburn, Susannah York and Robert Culp. Second of two consecutive films where child actor Simon Harrison got kidnapped with his mother and sister, after "The Wind and the Lion" (1975). The kidnapped daughter was the child of a Navy seviceman stationed at the Nea Makri base north of Athens. That base is now closed. Several base personnel and dependents assisted the cast and crew of the movie by providing or helping locate needed props and supplies.

The rescue sequences were filmed in the monastery located at Meteora in Greece. Its the same famed region that featured the finale of the later James Bond film "For Your Eyes Only" (1981) was also set later.

Sandy Howard (August 1, 1927 – May 16, 2008) was an American film and television producer. A native of the Bronx, New York City, Howard wrote short stories for publication in magazines like "Liberty", and worked as a publicist for Broadway shows until he became a director for the "Howdy Doody show"at the age of nineteen; he later produced the "Captain Kangaroo show".Howard teamed with Ray Heatherton creating the TV show "The Merry Mailman" in the early 1950s. He was producer of "The Barry Gray Radio Show" (1951–1958). In the 1960s, Howard moved to Hollywood to pursue a career in film. Among his film productions are "A Man Called Horse" (1970) and "Man in the Wilderness" (1971).

After an explosion on the set of this film, which Howard was producing for 20th Century Fox, a Greek electrician died and 11 others were injured in an explosion accident. Ironically, no one was seriously injured in the aerial scenes. Producer Terry Morse, Jr. was arrested by the Greek government, executive producer Sandy Howard was arrested for manslaughter, and detained in Greec, unable to leave the country for five months. Finally, a $250,000 out-of-court settlement was reached, which one Variety article called a "bribe", with the Greek government to release the film producer imprisoned by the military regime. While stuck in Greece, Howard claimed he made unfavourable deals to make films such as "The Return of a Man Called Horse" (1976) and "The Island of Dr Moreau" (1977)". Howard contributed as writer on "Vice Squad" (1982), which he produced for AVCO Embassy Pictures. He developed Alzheimers and spent the last ten years of his life as a resident at the Motion Picture & Television Fund Hospital in Woodland Hills, in the Alzheimer's and dementia care facility, Harry's Haven. He died in 2008.

Howard hired Jack Hill to write a sequel. Hill said, "I pitched them my idea, which they thought was good, and I wrote the script. Well, it turned out that the movie was a big flop and no one could understand why. I knew why, it was because they had the theory that it should be wall to wall action and there is nothing more boring." Hill then wrote "City on Fire" (1987) and "Death Ship" (1980) for Howard.