Set in the north Atlantic, off Nova Scotia, deep deep beneath the ocean waves, Director Dr. Andrews (Walter Pidgeon) is trying to keep his Project Neptune team's oceanographic research going aboard the OceanLab II, a sea base located near a dark abyss, in spite of opposition from Foundation Head, Dr. Norton Shepherd (Ed McGibbon).

On the ocean floor, a small team of scientists led by Dr. Hal Hamilton (Michael J. Reynolds) prepare to leave their underwater laboratory, about to return to the surface, after an extended stay performing research on undersea earthquakes.

Ironically, a huge underwater earthquake interrupts the Marine scientists' plans. The undersea lab they are all living in gets hits by an earthquake, which rips the lab loose from its moorings, and the cracked sea floor sends their lab careening down the underwater mountain it was built on, and plummeting into the abyss, taking the team with it, who only have seven days of air.

Trapped too deep for divers, the only chance for the rescue of the trapped Hamilton and crew is to enlist a modern experimental U.S. Navy mini-sub to dive in an attempt to survey the damage and photograph it for insurance purposes.

The sub is piloted by the arrogant Cmdr. Adrien Blake USN (Ben Gazzara). He heads up the rescue operation, and brings along Chief Diver "Mack" MacKay (Ernest Borgnine), Diver Bob Cousins (Donnelly Rhodes), and Dr. Leah Jansen (Yvette Mimieux), Hamilton's fiancé.

But, with Mack and Jansen onboard, the mission turns into an operation to rescue the scientists. Time is of the essence.

Blake finds the lab has been ripped from its moorings and has tumbled down an unexplored, deep ocean trench, presumably intact. With the lab's reserve air supply dwindling, the team descends into the unexplored trench and finds an incredible ecosystem populated with monstrously oversized fish.

After enormous amounts of exploration they find the lab partially intact. The rescue sub makes a few exploratory test dives to assess the situation, but are discontinued once the minisubmersible encounters dangerous situations.

Dr. Jansen forces the minisubmersible to dive well below the safe depth. The submersible's journey into the unknown discovers that Sealab fell into an unexplored cave. Inside that cave are numerous realistic dangers like strong underwater currents, limited battery power, and sand.

The rescue team finally enters the cave and encounters its share of fantasy challenges, like huge prehistoric fish, and find the surviving scientists breathing from scuba tanks and fending off giant, hungry eels. Diver Moulton sacrifices his life distracting the eels in order to enable the others to be rescued. The submarine returns to the surface with the two rescued scientists.

A 1973 science fiction film also known as "The Neptune Disaster," the movie has a subtitle of "An Underwater Odyssey,' directed by Daniel Petrie, produced by Sandy Howard, the film was based on an original story by writer Jack DeWitt, featuring underwater cinematography by Paul Herbermann, starring Ernest Borgnine (“Cabbie” from ESCAPE FROM NEW YORK), an Oscar winning actor, Ben Gazzara (“Brad Wesley” from ROAD HOUSE), earned multiple Golden Globe and Emmy nods and Broadway accolades. Walter Pidgeon had two Oscar nods in his pocket; Yvette Mimieux worked consistently as one of MGM Studios’ leading contract players throughout the 1960s and made her debut in H.G Wells "The Time Machine" (1960).

Sandy Howard, a film producer from the United States, brought the idea of "The Neptune Factor" to David Perlmutter and Harold Greenberg, who chose to produce the film. Howard wanted the film to be made in the United States, but Greenberg was able to have the film shot in Canada. The Canadian Film Development Corporation contributed 10% of the film's budget under the demand that Daniel Petrie be the director.

Producer Sandy Howard consulted a large number of marine biologists, oceanographers, and ichthyologists two years prior to taking the script into production. More than forty technical consultants from the Canadian Armed Forces, the Bedford Institute of Oceanography, and the Royal Ontario Museum as well as representatives from industries involved in various aspects of the undersea world participated in the making of this film. The nature of the Oceanlab underwater facility bears a resemblance to real-world projects of the 1960s such as the ConShelf Two project of Jacques Cousteau, NASA's NEEMO, and the US Navy SEALAB. The film's special effects utilized underwater photography of miniatures with actual marine life.

TV Guide gave this Irwin Allen paint-by-numbers underwater disaster movie made by 20th Century Fox and co-released by Fox and MGM Studios, one out of 5 stars, stating that "while its underwater photography was well done, the film was predictable, the characters stereotypes and the story lacking."

The New York Times also praised the photography, but found little else of value in the film.